Ceramic and glass technology
third stage
Fiber Process
 Glass fiber production tools are very different when considering optical
fibres, rovings or glass wool. Processes differ in the way drawing force is
applied and three drawing modes are distinguished:
Process
Tensile drawing
Articles
Optical fiber (using a preform), fibers for
reinforcement
Centrifugal drawing Fibrous glass for thermal insulation, acoustical
(or spraying
insulation, air filtration
tensile drawing,
gas friction drawing, and
centrifugal drawing.
 Nowadays, processes employ either tensile or centrifugal drawing gas
friction being combined with centrifugal drawing.
 Generally glass industry can be divided into the following types according to
manufacturing process and to application.
continuous fibers 3-13 𝜇m in diameter, which after textile processing serves
as electrically insulating fabrics and as reinforcement for organic polymers
(glass- reinforced plastics);
filtration cloths (fiber diameter 5 - 12 𝜇m) And chemically bonded
insulating mats for the building industry (12-30 𝜇m);
wool for thermal and acoustic insulations (20-30 𝜇m);
microfibers I - 3 𝜇m in diameter (even submicron fibers) made by double
drawing for special filtration and insulating purposes.
Tensile Drawing
 The process starts with molten glass delivered through a platinum heated
orifice plate. The number of orifices varies from 200 to 8000 with diameters
between 1 and 2.5mm. Glass flows through each orifice and is drawn
downwards.
 The drawing speed determines the glass filament diameter. Common speeds
are between 10 and 50 ms -1 allowing one to achieve fibre diameters ranging
between 5 and 25 mm.
 When drawn from platinum bushings The fibre diameter d is related to the
speed of drawing as follows: d ~ 1/𝜐 and The amount of glass flowing
through a nozzle q depends only slightly on the drawing speed :
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Ceramic and glass technology
third stage
 where q is the volume rate of flow, h is the height of glass above the nozzle,
r is the internal nozzle radius, l is the nozzle length, v is kinematic viscosity
of glass in the nozzle and k is a constant.
 Rovings are produced by combining several hundred fibres (range 100–2000
filaments). Several fibres are drawn in parallel and gathered as shown in Fig.
1.
 Antiabrading treatment is then essential for keeping excellent mechanical
resistance. Surface treatment also called sizing is required for adhesion to
the matrix in the case of composites. Figure 1 illustrates the manufacturing
of multi-fibres and the application of sizing. In a typical fibre-forming
machine the coating applicator is located just below the fibre-forming
bushing.
 Glass fibres leave the forming die at a temperature over 1000 °C and are
rapidly quenched to deposition temperature (at 100 °C) by the aqueous
sizing solution. Since the fibres are drawn under high speeds only short
times (about 5ms for a drawing speed of 1000mmin -1) are available for
applying the sizing. The filaments are then formed into a multifilament
strand and wound into a package.
 A number of compositions are used to produce fiberglass. Insulation fibers
are produced from modified soda-lime-silica compositions, which contain
more alumina and iron than container glasses. Most of the continuous
fiberglass is produced from so-called E glass, S (for strong) glass and C (for
corrosion-resistant) glass. S glass contains more alumina than E glass, while
C glass contains more silica and less alumina than E glass.
 Coatings play an essential role in the manufacturing of products made from
glass fibre. Because of its abrasiveness, glass fibre would be useless if not
coated during the sizing operation. In fact, without the protective coating the
fibre loses its strength and destructs .
 Sizing not only increases durability, but also adds significant value to the
product. Although glass composition is tailored according to applications,
sizing is the most important factor in differentiating one fibre product from
another. The wide variety of fibre glass applications leads to more than 100
sizing compositions on the market, each containing from 2 to 10
components.
 All sizings are aqueous chemical solutions containing 0.05–10%solids.Water
has a key role in the technology. In fact, while it carries and dilutes solid
components, it also cools the hot fibre allowing for the deposition of solids.
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Ceramic and glass technology
third stage
These are composed of the film former (polymer), the coupling agent
allowing adhesion of the film with the fibre, lubricants, antistatic compounds
and antioxidants.
Fig 1
 Optical fibres require refractive index gradient necessary to propagate the
optical beam. Different routes exist to generate a fibre with either
discontinuously or continuously varying index. When a discontinuously
changing index is to be fabricated, a preform can be made assembling an
internal core of larger refractive index and an external tube made of a glass
of lower refractive index. The diameter of the rod should be just slightly less
than the inner diameter of the tube. After placing the rod inside the tube, the
ensemble is heated to fuse the two glasses to form a unit. This preform is
then drawn to produce very fine fibres (Fig. 2). Again the fibres are
protected against abrasion by a polymer membrane.
 a double crucible can be used to produce the two glasses that are further
drawn to form the fibre.
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Ceramic and glass technology
third stage
Fig 2
Centrifugal Drawing
 Fibres used in glass wool and mats have to be chopped. Also, the surface
treatment will control the performance in use. The process again starts with
the glass delivery from a furnace.
 Glass with low viscosity melt is dropped onto rotating cylinders spinning at
an elevated velocity around their horizontal axis. Glass drops are projected
from one cylinder to another before producing fibres of different lengths and
reaching a moving belt downwards where remaining glass drops are aspired.
Meanwhile the fibres are sprayed with organic binders with curing
happening in the polymerization furnace downstream (Fig 3).
 the TEL process which supplies most of its glass wool production. The main
element is the centrifuge that spins around a vertical axis at 3000 rotations
per minute (Fig 4). It is supplied by a unique glass stream. Its flank presents
large holes through which glass drops are projected against a circumferential
and inclined wall presenting numerous smaller holes (10 000 about 1mm in
diameter). Centrifugal drawing is assisted by annealed air blowers (Fig.
10.20). On reaching the moving belt the fibres are sprayed by polymers
(protecting the fibres from erosion) further cured downstream. Moving belts
allow for a continuous process delivery.
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Ceramic and glass technology
third stage
Fig 3
Fig 4
Annealing
 Annealing of glass is a controlled heat treatment designed to prevent or
remove undesirably large internal stresses in glass-ware. Spontaneous stress
relief takes place in glass by viscous flow within the annealing range
limited by the upper and Lower annealing temperature*; for purposes of
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Ceramic and glass technology
third stage
comparison the temperatures are characterized by viscosities of 10 12and 1013
pa.s. At the upper annealing temperate internal stress is substantially
relieved within a few minutes, at the lower annealing temperature within a
few hours.
 Formation of internal stress may be illustrated as follows: when a glass plate
cooled or heated in a temperature range where the glass behaves as a solid ,
elastic body (i.e. below the lower annealing temperature) a temperature grad
arises between the surface and the internal plate layers.
 as no free expansion or contraction of these layers can occur according to
the respective thermal coefficient, mechanical stresses are created;
compression and tensile stresses mutually compensated.
 The stress is proportional to the temperature differences and the expansion
coefficient; maximum stress values at the centre and surface are
 where E is the elasticity nodules, ∝: is the coefficient of thermal expansion,
𝜇 is Poisson’s ratio, T is the temperature at the plate centre or surface and
Tav is the average temperature corresponding to the overall sample size.
Similar relationships are available for a variety of geometrical shapes. As
soon as the temperatures in the plate are equalized, the type of stress
considered so far is relieved, and is therefore called the temporary stress
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Ceramic and glass technology
third stage
 If the glass is cooled from sufficiently high temperatures where it is still
formable, temperatures gradients but no stress will arise as the latter will
immediately be relieved by viscous flow. For a constant rate of cooling, it
may be assumed that the stress will appear only after complete solidification
of the glass at the moment when temperature differences between glass
surface and interior begin to equalize.
 From this moment. the compression stress will increase on the surface and
internal stress inside.
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Ceramic and glass technology
third stage
 The stress will reach a maximum when the temperatures have completely
equalized and will remain in the article as a permanent stress. Its distribution
in a plate is again parabolic, but it acts in a direction opposite to that of
temporary stress and corresponds in size to the stress released by viscous
flow where the temperature gradient was formed.
 Permanent tensile stress will generally arise at points which have been
maintained at the highest temperature during cooling. And compression
stress will appear at points cooling at the highest rate
 The occurrence of permanent stress is thus related to glass solidification in
the presence of temperature gradients
 The danger of temporary stress lies in the fact that too fast cooling may
bring about fracture when the tensile strength of glass has been exceeded,
permanent stress may lead to the same consequence in the final stage of
temperature equalization
 Badly distributed permanent stress, which always includes its tensile
component: will reduce the final strength of the ware and thus impair its
value.
 The occurrence of both temporary and permanent stress could be avoided
cooling down the glass at such a low rate as to render the temperature
gradient negligible
 however, such a procedure is un acceptable in practice and the aim is find a
way of cooling which would allow, with the highest economy, the
production of ware with permanent stress within permissible limits, The
glass annealing process which meets the above requirements, may be
regarded as consisting of several stage
Heating the ware. when it has cooled to a lower temperature, to the
upper annealing temperature.
Holding at the upper annealing temperature for a period required for
elimination of temperature gradients and release of the stresses present.
Slow cooling within a temperature range in which glass has a viscosity
of 1012and 1013 pa s to suppress the formation of permanent stresses.
more rapid cooling to ambient temperature at a rate Limited by the
formation of temporary stress.
 The rate of stress relief is well approximate by an empirical equation' as
suggested in 1920 ,by Adams and Williamson or1 the basis of their research
of optical glass annealing:
𝑑𝜎
𝑑𝑡
or after integration
= A𝜎 2
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Ceramic and glass technology
third stage
 where A is annealing constant which depends on glass composition
(viscosity) and is an exponential function of temperature. The AdamsWilliamson equation has been used as a basis for calculating annealing
schedules in industrial practice.
 Determination of the annealing constant and its temperature dependence for
a given glass is time consuming and not really necessary; practical use is
made of the fact that high initial stresses are relieved to harmless levels
within several minutes at a viscosity of 1012 Pa. s corresponding to the upper
annealing temperature. Glass is conditioned at this temperature for a period
of time required for relieving the stress and eliminating the temperature
difference (the usual period required being 5 - 20 minutes).
 If no precise data on viscosity are available, the upper annealing
temperature can be estimated by adding 5-10 C to the Tg (transformation
temperature) determined from the curve.
 The second stage of the annealing process involves cooling in the annealing
range. The cooling rate should preclude the formation of permanent stresses
The relations between cooling rate and stress have the general form
 where Ma is a material constant (depending on glass composition and
temperature h is the cooling rate 𝛿 is the effective dimension; full plate
thickness when cooling from one side, half thickness when cooling from
both sides; 𝛿 = r for full sphere, cylinders, while 𝛿equal to the wall thickness
(r2-r1) for hollow articles.
 𝜁 is dimensionless quantity which is a function of article shape and size; it is
obtained solutions of heat transfer in bodies of various symmetries. The
value of 𝜁 can be calculated on simple shapes from the heat transfer laws,
and determined experimentally for more complex shapes.
 The values Ma and 𝜁 are then used for calculating the maximum permissible
cooling Rate h1 within the annealing range according to eqa:
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Ceramic and glass technology
third stage
 In order to facilitate control of the specified temperatures, the annealing
schedule can be adjusted so that lower cooling rates are applied at the
beginning h=2/3 h1, rate h1 in the second stage and h =2 h1 in the last. The
overall residence time in the annealing range remains unchanged.
 Below the lower annealing temperature, the rate of stress relief in glass is
already low that permanent stress can no longer occur. The glass can then be
cooled at a third to six-fold rate.
 The annealing schedule is represented graphically by a curve which show
the respective lehr temperatures and the calculated cooling rates. The dashed
line shows the theoretically calculated schedule while the full lines represent
the cooling employed in practice with respect to the possible temperature
control the lehr.
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Ceramic and glass technology
third stage
Tempering
 Annealing of glass is a process intended for the elimination of stress,
However suitably distributed permanent stress may substantially increase
the strength of glass Therefore, in some cases, glass is intentionally cooled at
a high rate so that compression stress arises in surface layers the process is
called thermal tempering or toughening
 Toughening carried out by quenching the glass from a temperature close to
the softening point using a stream of air from suitably arranged nozzles, or
immersion of the object in oil bath or by pressing it in contact with metal
surfaces.
 Toughened glass has superior strength, as fracture results from the effect of
tensile (bending) stressing of the surface, If compression stress has been set
up in the surface layers, any external force must first overcome this
"prestress" before tensile stressing becomes effective. The conditions in a
toughened glass plate subjected to external bending stress are show in l4; a
permanent internal stress acts against the external force.
 The degree of toughening is characterized by the specific optical path
difference* ∆, corresponding to the maximum tensile stress inside the object
wall 𝜎𝑐 here, the relationship holds
 where ∆ is the path difference in nm cm-1, B is the stress-optical coefficient
( 2.5 x l0-12 Pa-1.), 𝜎𝑐 is tensile stress in Pa .
 In a toughened plate, the compression stress at the surface is roughly equal
to double the tensile stress in the centre plane.
 In practice, the degree of toughening is expressed by a relative quantity
obtained as a multiple of wavelength corresponding to one order ∆𝑟𝑒𝑙 =
∆/540; its numerical value varies in the range 1 to 6.
 A significant quantity is the so-called toughening temperature: when cooled
from this or a higher temperature. permanent stress at a given cooling rate
reaches its ;maximum value. Further increase in initial temperature therefore
does not change the degree of toughening which depends solely on cooling
rate. The toughening temperature for several glasses is shown in Fig.
corresponds to the point where the curve bends into the horizontal section.
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Ceramic and glass technology
third stage
 Toughening of glass is most frequently used with sheet glass intended for
vehicle wind screens, building items (e.g. all-glass doors) etc. Toughening is
also employ in the manufacture of domestic glass-ware.
 Toughened glasses cannot used at elevated temperatures. since relaxation of
great stresses takes place from 300-400 C upwards.
Chemical Tempering of Glass
 Chemical tempering is applied for special applications like aeroplane and
high speed train windshields, requiring high safety levels, and for thin
glazing for which conventional thermal tempering would mark the surface or
would not allow for sufficient strengthening. Although production yields are
much less important,
 chemical tempering has to be used to achieve very high level of stresses
required in the range of 500–600MPa. The treated depth of glass (diffusion
length) is reduced to hundreds of micrometers.
 The glass to be treated is immersed in a salt bath containing cations the radii
of which are larger than those to be exchanged in the glass.
 Ion exchange induces compression at the glazing surface. Generally, the
cations are alkali metal ions since they diffuse in glass much more rapidly
than any other ions . For instance, a glass containing sodium is strengthened
by immersion into a potassium salt bath (generally KNO3). To enhance
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Ceramic and glass technology
third stage
diffusion elevated temperatures (400–500 8C) are used for the treatment.
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