Automation of Vacuum Pressure Impregnation process of Insulation for an Air cooled Turbo generator DEPT. OF EEE 1 AITS-HYD 1. INTRODUCTION Electrical insulating materials are defined as materials that offer alarge resistance to the flow of current and for that reason they are used to keepthe current in its proper path i.e. along the conductor. Insulation is the heart ofthe generator. Since generator principle is based on the induction of e.m.f in aconductor when placed in a varying magnetic field. There should be properinsulation between the magnetic field and the conductors. For smaller capacitiesof few KW, the insulation may not affect more on the performance of thegenerator but for larger capacities of few MW (>100MW) the optimization ofinsulation is an inevitable task. Moreover the thickness of insulation should beon par with the level of the voltage, also non homogenic insulation provisionsmay lead to deterioration where it is thin and prone to hazardous short circuits. Also the insulating materials applied to the conductors are required to be flexibleand have high specific (dielectric) strength and ability to withstand unlimitedcycles of heating and cooling. Keeping this in view among other insulating materials like solidsgases etc liquid dielectrics are playing a major role in heavy electrical equipmentwhere the can embedded deep into the micro pores and provide betterinsulating properties. Whereas solid di-electrics provide better insulation withlower thickness and with greater mechanical strength. So the process ofinsulation design which has the added advantage of both solid and liquiddielectrics would be a superior process of insulation design. One such processwhich has all the above qualities is the VPI (vacuum pressurised impregnation)process and has proven to be the best process till date. Vacuum impregnation as an industrial process has been in commercial use for more than 60 years. For the world’s largest manufacturers, it continues to be the preferred process through which to guarantee the pressure-proof, leak-proof, and corrosion-proof requirements of parts and components in critical operations.
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Automation of Vacuum Pressure Impregnation process of Insulation for an Air cooled Turbo generator
DEPT. OF EEE 1 AITS-HYD
1. INTRODUCTION
Electrical insulating materials are defined as materials that offer alarge resistance to
the flow of current and for that reason they are used to keepthe current in its proper path i.e.
along the conductor. Insulation is the heart ofthe generator.
Since generator principle is based on the induction of e.m.f in aconductor when
placed in a varying magnetic field. There should be properinsulation between the magnetic
field and the conductors.
For smaller capacitiesof few KW, the insulation may not affect more on the
performance of thegenerator but for larger capacities of few MW (>100MW) the
optimization ofinsulation is an inevitable task.
Moreover the thickness of insulation should beon par with the level of the voltage,
also non homogenic insulation provisionsmay lead to deterioration where it is thin and
prone to hazardous short circuits.
Also the insulating materials applied to the conductors are required to be flexibleand
have high specific (dielectric) strength and ability to withstand unlimitedcycles of heating
and cooling.
Keeping this in view among other insulating materials like solidsgases etc liquid
dielectrics are playing a major role in heavy electrical equipmentwhere the can embedded
deep into the micro pores and provide betterinsulating properties.
Whereas solid di-electrics provide better insulation withlower thickness and with
greater mechanical strength.
So the process ofinsulation design which has the added advantage of both solid and
liquiddielectrics would be a superior process of insulation design.
One such processwhich has all the above qualities is the VPI (vacuum pressurised
impregnation)process and has proven to be the best process till date.
Vacuum impregnation as an industrial process has been in commercial use for more
than 60 years.
For the world’s largest manufacturers, it continues to be the preferred process through
which to guarantee the pressure-proof, leak-proof, and corrosion-proof requirements of
parts and components in critical operations.
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1.1NECESSITY
During normal operation,Electrical machines like Generators, Transformers etc are
subjectedto the failure of the insulation system. How long an insulation system will be
serviceable depends on the materials chosen and the service environment.Thermal,
mechanical, voltage and environmental stresses all combine to reduce the service life of the
Electrical machines.
a) To reduce this losses, We have to manufacture all Electrical machines like
Generators, Transformers using the Vacuum Pressure Impregnation (VPI) process.
b) This system strengthens the insulation system and extends the service life of the
transformer. The VPI process is the most advanced system in use today.
c) VPI includes pressure in addition to vacuum, thus assuring goodpenetration of the
varnish in the coil.
d) The result is improved mechanical strengthand electrical properties.
e) With the improved penetration, a void free coil isachieved as well as giving greater
mechanical strength.
1. 2OBJECTIVE
The ultimate goal of vacuum impregnation is to seal leak / migration paths without
impacting the functional,assembly or appearance characteristics of a part. Functional
characteristics include the ability for fluids or gasses to flow only where needed in order to
enhance in-service performance of the components’ design. Assembly characteristics, which
must be maintained, include performance of tapped holes; the integrity of mating and
sealing surfaces; the elimination of residual internal contamination in water jackets; sockets;
surfaces; and dimensional areas. Appearance characteristics include oxidation and
discoloration.
The processing methods may be used to variety of impregnate parts. The method
selected depends on the sealant and the requirements of the parts. Fundamentally, vacuum
impregnation sealing of porosity addresses a pair of fluid mechanics problems. The laws of
fluid mechanics govern the flow problem of removing the air from the pores and the flow
problem of filling the pores with liquid sealant.
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1.3 THEME
To insulate winding elements in rotating high voltage electrical machines the
impregnation technique based on vacuum pressure impregnation has become very popular in
recent years, In doing so, the winding elements for construction engineering reasons are made
either as preformed coils or conduct and scolds preferably performed bars. The winding
elements are provided with mica-containing mail solution and are further tired in a vacuum
pressure impregnation process.
The main theme/characteristics of this insulation system are:
a. Better heat transfer resulting from penetration into minute air
gaps in between laminations and bar insulation.
b. Low dielectric loss resulting in increased life of insulation and so
the machine.
c. High resistance against the effect of moisture.
d. Reduction of time cycle of insulation.
1.4 ORGANISATION
In this project documentation we have initially put the definition and objective of the
project as well as the design of the project which is followed by theimplementation and
testing phases
Chapter 1: It explains about the introduction to the project, necessity of the project.
Chapter 2: This chapter explains the Literature survey about VPI.
Chapter 3: This chapter explains about development of the VPI.
Chapter 4: This chapter explains the analysis of the VPI process for the stator of Turbo-
generator
Finally the project has been concludedsuccessfully and also the future enhancements
of the project were given in this documentation.
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2. LITERATURE SURVEY
2.1 INTRODUCTION
The VPI process is the most effective way known to eliminate the dead air spaces that
cause hot spots within the Electric Machines like large generators and transformer coils.
These hot spots can be 20* higher than the average coil temperature. The VPI process, along
with a good resin, provides a low thermal resistance path that lowers the average operating
temperature of the Machine.
During the VPI process, the resin seals the machine against environmental conditions
and bonds all components of the insulation system together for good mechanical strength.
This is very effective in reducing mechanical vibrations. This greatly reduces the audible
noise level of the generator. The VPI process and resin also enhances the dielectric capability
between windings and between the windings and ground. This allows the transformer to
survive higher voltage stress levels without failure.
2.2 EXISTING SYSTEM
Electrical machines quality is highly dependent on the vacuum pressure impregnation
insulation system. All the high voltage machines, pole coils irrespective of size and shape are
being impregnated under vacuum and pressure of self-developed resin systems. The stringent
quality tests on the resin mixtures and strictly following the vacuum pressure impregnation
and systematic cooling and heating cycle of resin mixture and sophisticated automatic control
systems made the insulation systems for better and better quality for more than 30 years. The
insulating materials used for wedges are resin poor and accelerator treated. For example Main
insulation tapes, mica paper tapes, overhang protective tapes, shrink tape and glass mates
HM693 are treated with accelerator.
After impregnation, they become hard and experiments were conducted for voltage
endurance at room temperature and at evaluated temperature continuously for more than 3
years. Though the mica tape can withstand 20kv per mm, the extrapolation has been done at
4kv/mm and life expectancy is around 100yrs. With an operation stress level of less than
4kv/mm, factor of safety is considerable. The Vacuum pressure impregnation system was
brought by Dr.Meyer with the collaboration of wasting house in the year 1956. The resins
used were of polyester. The mica tapes used for Vacuum pressure impregnation systems are
ROGS 275, ROGS 275.1 and ROV 292. ROGS 275 tapes are with glass cloth baking up to
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13.8 kV voltage levels ROV 292 mica paper tapes are with polyester fleece above and more
penetration of resin. ROGS 275.1 tape is special glue varnish for tropical countries like India
and Brazil to resist higher humidity. The glue being used for main insulation tape is X2026
and for conductor insulation is X2027.
The resin used for Vacuum pressure impregnation is ET 884, a mixture of epoxy resin
E1023 (lekuther m x 18) and hardener H1006 in 1:1.2 ratio by weight. In kwu, the
components are mixed in 1:1 ratio.
E1023: The resin is in drums of 220 kgs weight. It is in crystal form at temperature of 14 or
20deg.C the container is resin is available in drum the reason is faster heating in furnacethe
resin in liquid state shall not come out of the container. The drums are kept in oven and
heated up to 1000
C for about 18hrs. If the resin is not fully in liquid condition, it can be
heated up to 1250C. The storage tank is filled with resin first depending on the volume and
ratio of mixture at a temperature of 600 C through hose pipes. Resin filling is being done by
creating 0.2 bar vacuum in the tank.
2.2.1 RESIN MIXTURE
The mixing ratio of resin to harden is 46:54 parts. The resin mixture required for the
Impregnation tank is 27000lts. A job of 1.9m height and 4.5m diameter can be impregnated.
a )Size of the tanks:
Main impregnation tank = 4.5m (pie) x 3.0m ht.
There are 3-inch vessels for different sizes jobs impregnation.
I. Vessel (1) – 3.8m pie x 2.25m ht.
II. Vessel (2) – 3.0m pie x 2.3m ht.
III. Vessel (3) – 2.0m pie x 2.3m ht.
Three Storage tanks of each resin capacity9000 litres are in the operation for storing.
The resin mixture cooling and heating cycle is by circulating the resin through the heat
exchangers. Oil heated by water is being used for heat exchangers.
The Vacuum pressure impregnation cycle is as per WIV 114.1 standard. The job is
kept in an oven for a period of 12hrs at a temperature of 70OC. Six no. of thermocouples are
inserted on the back of the core and measured the temperature. Job insertion in the
impregnation tank is at 70oc. The lid of the impregnation tank will be in open condition. The
vessels are kept clean. Resin available is wiped out by Methylene. Traces of resin shall not be
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allowed on the inner side of the tank. It reacts with humidity and scale formation will takes
place. These components obstruct the filters also. The resin at the time of cleaning is
carefully removed by wiping with rubber sheets. Keeping the vessel in slant position on the
ground also cleans the inner vessels. After ensuring the perfect cleaning, the tank should be
allowed for further operation. The job is inserted in the tank the temperature monitoring
thermocouples are placed on the back of the core. The lid is allowed to come down by
hydraulic motor. Silicon grease is applied on the surface of tank where the lid is touching.
A rubber gasket is also provided on the rim not to allow any leakage. Air pipes are closed and
vacuum pumps will be started.
b)Vacuum creation – 0.35 torr for 2 hrs:
The job temperature is to be maintained always above 65OC, if found less, tank can be
heated up. In practice, the vacuum can be created in 2 hrs. Siemens adept before starts of 2nd
shift (3.0 pm), they create 0.35 torr vacuum and it will be continued till next day morning 1st
shift (6.00 am) min. requirement is 2hrs.During this time the resin cooling is being carried
out to reach 10deg.C and heated up automatically to 70deg.C.
2.2.2 IMPREGNATION
The resin mixture is to be heated to 70deg.C. Every day morning a 20ml sample will
be taken to laboratory tests. Viscosity will be measured at 70deg.C. It should not be more
than 45 CD +10%. Anew resin will beat 15 CP. New resin and hardener mixture is to be
added if the viscosity is more. The resin filling is being completed in 25 minute. At this time,
the vacuum reduces to 0.5 Torr – 1 Torr level. The resin is to be allowed to settle for 15 min.
The level of resin is above 100mm over the job.
a) Pressuring – 3 bar:
With the hydrostatic pressure of the resin, only surface of the insulation can be filled with
resin. To have an effective penetration up to the end of a barrier, pressure is to be created to 3
bar (2 bar over atm. Pressure of 1 bar).
b)Gelling time:
The polymerization of resin and accelerator take place at this time. At 65OC, the time
required is 170 min. The insulation gets hardened.
c) Curing – 14 hour at 140 O
C:
The resin is to be pumped back to the storage tank. The job is to be removed from the tank
and allowed for dripping. It is kept in oven at 140deg.C for min of 14 hrs. The accelerator
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B1057.1 is to be placed at 4 corners of the oven. In curing process the accelerator vapors will
react with surface resin and cures.
2.3 DISADVANTAGES OF EXISTING SYSTEM
a. If any short circuit is noticed, the repairing process is difficult and need of excess
resin from outside.
b. Dependability for basic insulating material on foreign supply
2.4 CONCLUSION
Hence Vacuum-Pressure Impregnation technology can be used in a wide range of
applications from insulating electrical coil windings to sealing porous metal castings. It
normally produces better work in less time and at a lower cost than other available
procedures.
These systems can be large or small, simple or highly sophisticated and equipped with
manual, semi-automatic or automatic controls. Vacuum Pressure Impregnation (VPI) yields
superior results with better insulating properties, combined with “flexible” rigidity, resulting
in greater overall reliability and longer life. VPI reduces coil vibration by serving as an
adhesive between coil wires, coil insulation, and by bonding coils to their slots.
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3. SYSTEM DEVELOPMENT
3.1. INTRODUCTION (Vacuum Pressure Impregnation)
VPI produces a better insulation system than can be obtained by conventional
methods, better environmental protection and superior chemical and moisture resistance
(salt water immersion tests).The removal of air voids from the windings assures longer
electrical life and less opportunity for corona. In addition, more solid fill means heat will be
conducted to the outside more efficiently, better thermal endurance, lower hot-spot
temperatures, and lower temperature rise. Further, VPI with a solvent less product provides
greater mechanical and structural strength and may eliminate the need for a surge ring.
Blocking or tying may be replaced with Dacron felt pads that will form the necessary
blocking when impregnated with resin.
Global vacuum pressure impregnation insulation system exhibits various merits in
insulation performance and reliability for operation including maintenance. Therefore, the
system is suitable for ordinary turbine generators, especially, the generators for geothermal
power plant. Developed a global vacuum pressure impregnation insulation system (F-resin/G
insulation system) for large size turbine generators and had put into practical use in 1993.
In the process of evaluation of insulation system, evaluated impregnating ability,
electrical, and mechanical characteristics. From evaluation able to achieve the improvement
of heat cycle, heat resistance and V-t characteristics against usual insulation system, because
of the utilization of internal electrical field relaxation layer, thermal stress relaxation layer,
insulation tapes with excellent impregnation ability and epoxy resin with high heat resistance.
As the insulation characteristics are affected not only utilized materials and their composition
but also manufacturing process, it is reasonable to suppose that we can obtain the
improvement of insulation performances and stabilization of quality by using the taping
simulation technique.
In the global vacuum impregnation insulation system, stator winding inserted into
stator is immersed in epoxy resin, and is impregnated under vacuum and pressure. Through
this VPI process not only the coil insulation is formed but also the resin penetrates into stator
slot clearance, wedges and coils. Additionally, all stator parts are coated by the resin.
Furthermore, the stator coil insulation is impregnated by the resin continuously andrigidly
from slot portion to coil end conductor connecting portion. Therefore, there are a lot of merits
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such as high reliability of winding, no looseness of stator core, corrosion proof, and
prevention of coil vibration as well as excellent heat conductivity from the conductor to the
core owing to filling up of stator slot clearance by the resin. Furthermore, replacement of
wedges is not required because no wedge looseness is induced. These features are suitable for
ordinary turbine generators, especially, the generators for geothermal power plant,
surrounded by atmosphere containing hydrogen sulphide Though this insulation system is
mainly applied to air cooled turbine generators with output range of from 20 to 260MVA, it
is also possible to manufacture hydrogen cooled turbine generators with output range of from
50 to340MVA, utilizing the merits of global vacuum pressure impregnation insulation
system.
3.1.1. FEATURES OF F-RESIN/G INSULATUION SYSTEM
Fig. 3.1 Appearance of 126MVA air cooled turbine generator stator.
Global vacuum pressure impregnation insulation system retains many merits such as
improvement of insulation performances, stabilization of quality, shortening of
manufacturing period and mitigation of maintenance load. In particular, since a thermal stress
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relaxation layer is provided in the F-resin/G insulation system, a stable heat cycle resistance
characteristic is realized and enables to serve for the DSS (daily start and stop) operation
mode which corresponds to current power demand.
The main features of the F-resin/G insulation system are listed below, and Figure 2 shows
cross section of winding.
a) Utilization of epoxy impregnation resin with long enduring life and high heat
resistance.
b) Utilization of main insulation tapes with excellent impregnation ability.
c) Utilization of internal electrical field relaxation layer
d) Utilization of thermal stress relaxation layer
3.2. Glossary of VPI terms:
1)Bond Strength:The measure of force required to break the bond of varnished helical coils
of enamelled magnet wire.
2)Bump: Briefly revert from vacuum to atmospheric pressure and again draw the vacuum.
Applied in the wet vacuum cycle to help dislodge trapped air and improve penetration.
3)Centipoise:Unit of viscosity. Usually measured by the drag on a turning spindle immersed
in the liquid, Brookfield viscosity. A force of 0.01 dyne per centimetre.
4)Film Build: Average build-up of cured resin on one side of a metal panel.
5)Copolymer: A polymer formed by the of the resin.
6)Deaerate:Remove air and other gasses by vacuum. Note that initial deaeration after a tank
fill can take from several hours to as much as 3 or 4 days depending on the amount, type and
condition of the resin.
7)Dielectric Constant:The property of a material that determines how much charge is stored
per unit volume when unit voltage is applied. The capacitance of a material compared with
the capacitance of an equal volume of air or vacuum.
8)Dielectric Strength: The voltage a material can withstand before breakdown occurs.
Usually expressed in “Volts Per Mil”. Interestingly, a thicker section of material has a higher
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total breakdown but a lower dielectric strength, i.e. dielectric strength for one mil Mylar tape
may be 3000 VPM but for 2 mils, breakdown would be only 5000 Volts (2500 VPM).
9)Dissipation Factor:An indication of energy loss in the circuit, as in the production of
unused heat. A multiplier used to obtain useful energy compared to supplied energy.
10Electrical Varnish:A resinous material used to protect and insulate electrical apparatus,
which is applied as a liquid and converted by chemical action, with heat or without, to form a
solid film or mass.
11) Flash Point: The temperature at which enough vapour is generated to flash if a spark or
flame is introduced.
12) Foaming: An accumulation of frothy bubbles caused under vacuum by the expansion of
air and other gasses trapped within the resin.
13) Form Wound: Describes a coil that is formed or shaped over a fixture. Often made with
rectangular conductors laid precisely together, interleaved with flexible insulation. Also
usually covered with one or several wraps of half lapped tape. Also a motor incorporating
such coils.
14)Green: Describes coils or devices that have not been treated, coated or sealed.
15)Half Lap: Spiral tape wrap in which each turn overlaps the previous one by a half tape
width. Provides a double thickness of tape.
16)Hertz: A term indicating the frequency of one cycle per second.
Hg: Chemical symbol for the element, mercury.
18)Holding Tank: A reservoir for keeping the varnish when it is not in use. Should be
equipped with heavy duty mixer and vacuum capability. Refrigeration may be needed in
warmer climates and/or where hot dipping or continuous use is anticipated. Also consider
cooling when infrequent use (low tank turnover) is anticipated. Storage @ <75°F is
suggested. Vacuum cycles can be short- ended by storing the resin under vacuum to prevent
build up of air and other gasses in the resin.
19)Millibar: A unit of atmospheric pressure: 0.75 mm Hg (75 microns). One mm equals 1.33
mbar.
Penetration and Fill: The process by which
the varnish is drawn or forced into and retained
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20)Preheat:To bake the device before processing.
21)Preheated Oven:Oven heated until the skins(inside walls) are at temperature and
temperaturehas stabilized. May take several hours.
22)psi :Abbreviation for “Pounds per Square Inch”.Random Wound: Describes a coil in
which thewires do not lie in an even pattern. Not shapedbefore insertion in the devise. Also a
motor containing such coils. Sometimes called “Mush Wound”.
23)Resins:A class of organic, liquid, fusible materials of synthetic or natural origin that are
polymeric in structure. Storage Life: The time during which a liquid resin can be stored @
70°F and remain suitable for use. Also called “Shelf Life”. See Tank Life.
24)Stress Crack: A fissure in the cured resin caused by unequal expansion and contraction
of the core, flexible insulation, resin, etc.
25)Tank Life:The time the product remains usable in service. Tank life is affected by the
frequency of use, processing temperature, turnover of material, storage temperature, and
occasionally by contaminants. Also called “Pot Life”. Thermal Conductivity: The ability of a
material to conduct heat. Usually expressed as: Calories/sec/cm2/°F/cm thickness.
26)Thixotropic (Thixotropy):Describes materials that liquefy or flow when agitated
(mixed) and return to a thick consistency when allowed to rest, e.g. ketchup. A thixotropic
material can therefore, be used at both high and low viscosities.
27)Torr: Unit of pressure (vacuum): 1 mm Hg
28) Vacuum Chamber:Vessel where devices are processed. May be equipped for both
vacuum and pressure. Usually also includes 2 portholes, the sight port and the light port, one
for illumination, the other for viewing the process .
29)VapourPressure: An indication of the evaporation rate. The pressure in an enclosed
container when the vapour and liquid are in equilibrium.
30)Viscosity:The resistance of a material to flow. Higher viscosity liquid flows more slowly,
lower more quickly. May be measured in centipoise, or in minutes and seconds.
31)Volume Resistivity: The ability of a material to resist the passage of electricity through
its bulk. The value is expressed in “Ohm-Cm”.
Resins: A class of organic, liquid, fusible materials of
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3.3. Typical VPI cycle...
This process should be varied according to the VPI equipment, resin and apparatus to
be treated. Equipment to be processed must be green (untreated), and tapes untreated, open,
or permeable so as not to block the resin. Resin filled or B-Staged tapes that cure with heat
should not be used.
A)Preheat:
The part is placed in an oven and heated to 250°-325°F. The preheat serves to
evaporate moisture and any volatile oils, which may be present. It also improves penetration
and fill by lowering resin viscosity surrounding the part, and creates suction when the part is
cooled by immersion in the resin. Before proceeding to the next step, cool to 150°F or cooler.
B)Dry Vacuum:
After placing the part in the vacuum chamber, apply vacuum, typically 1 - 4 mm Hg,
for 30 minutes. During this phase, air and any remaining moisture, oil, etc. is removed.
NOTE: During the dry vacuum, the resin in the holding tank should be deaerated and
thixotropic products should be agitated (mixed) in the holding tank for at least 15 minutes.
Agitation will reduce viscosity for effective penetration and fill.
C)Wet Vacuum:
Immediately after mixing, introduce the resin into the vacuum chamber allowing it to
flow up from the bottom so as not to block further penetration. The resin should cover the
part by a depth of at least 1 inch. If excessive foaming occurs during the vacuum process,
slow down the introduction of resin to allow time for air and gasses to escape. Maintain
recommended vacuum for 20-60 minutes. Larger units and those with more layers of tape
will require a longer time under vacuum. For fine wire coils and constricted parts, bumping
the vacuum may increase penetration.
3.4. Pressure Cycle:
When the wet vacuum portion of the cycle is complete and the parts are still totally
immersed, pressurize to 90 – 100 psi with air for an hour or longer. Note: Depending on resin
characteristics, an inert gas may be required to bring vacuum up to atmospheric pressure.
Form wound devices will require about 15 minutes per half lap of tape. Release pressure.
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Thixotropic products will have a higher build if allowed an atmospheric soak for 30-60
minutes.
a)Removal and Drain:
Vent pressure and remove the part or drain the resin. A removal rate of 4 inches per
minute or slower should be used so that the resin forms a uniform coating.
Drain may take place over the tank so that runoff can be captured and returned to the
reservoir. While draining, the part should hang at an angle so that flat surfaces can drain
readily.
This will tend to eliminate thick sections, which might promote stress cracks. Drain
until major runoff stops. Follow specific recommendations on the product data sheet.
If using a thixotropic product, allow a period of 1-2 hours or more after drain to
promote resin retention during cure. Thixotropic products should show minimal drain in the
oven.
b) Bake: Place the treated part in a fully preheated oven. Cure using DOLPH’
recommendations for time and temperature according to the product data sheet.
Applications for VPI...
1) High Voltage Machines
2) High Temperature Apparatus
3) Transformers
4) HID Ballasts
5) Random Wound Stators
6) Chemical Duty Motors
7) Rugged Duty Motors
8) Inverters
9) Form Wound Coils
10) Armatures With Coils Installed
11) Precision Wound Transformers
12) Ferro-Resonant Units
Bake:
ake:
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3.5 EPOXY RESINS:
Epoxy resins are poly ethers derived from epi-chlorohydrin and Bis-phenol monomers
through condensation polymerization process. These resins are product of alkaline condensed
of epi-chlorohydrin and product of alkaline condensed of epi-chlorohydrin and poly-hydric
compounds.
In epoxy resins cross-linking is produced by cure reactions. The liquid polymer has
reactive functional group like oil etc, otherwise vacuum as pre polymer. The pre polymer of
epoxy resins allowed to react curing agents of low inductor weights such as poly-amines,
poly-amides, poly-sulphides, phenol, urea, formaldehyde, acids anhydrides etc, to produce
the three dimensional cross linked structures.Hence epoxy resins exhibit outstanding
toughness, chemical inertness and excellent mechanical and thermal shock resistance. They
also possess good adhesion property. Epoxy resins can be used continuously up to 300F, but
with special additions, the capability can be increased up to a temperature of 500F.
Epoxy resins are made use as an efficient coating material. This includes coating of
tanks containing chemicals, coating for corrosion and abrasion resistant containers. Epoxy
resins are made up of as attractive corrosion and wear resistant floor ware finishes.
These are also used as industrial flooring material. They are also used as highways
Surfacing and patching material. Moulding compounds of epoxy resins such as pipe fitting
electrical components bobbins for coil winding and components of tooling industrial finds
greater application in industries.The epoxy resins similar to polyester resins can be laminated
and Fibre Reinforced (FPR) and used in glass fibre boats, lightweight helicopters and
aeroplanes parts.
In the modern electronic industry, the application of epoxy resins is great. Potting and
encapsulation (coating with plastic resin) is used for electronic parts. Most of the printed
circuits bodies are made of laminated epoxy resin which is light but strong and tough.
3.5.1 PROPERTIES:
1. Epoxy resins have good mechanical strength less shrinkage and excellent
dimensional stable after casting.
2. Chemical resistance is high.
3. Good adhesion to metals.
4. To impact hardness certain organic acid anhydrides and alphabetic amines are mixed.
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3.5.2 APPLICATIONS:
1. They are used in the manufacture of laminated insulating boards.
2. Dimensional stability prevents crack formation in castings. They are also used as
insulating varnishes.
Epoxy resins are polyether resins containing more than one epoxy group capable of
being converted into thethermoset form. These resins, on curing, do not create volatile
products in spite of the presence of a volatile solvent. The epoxies may be named as oxides,
such as ethylene oxides (epoxy ethane), or epoxide. The epoxy group also known as oxirane
contains an oxygen atom bonded with two carbon atoms, which in their turn are bound by
separate bonds as in Scheme I:
Fig.3.2 Seheme I
The simplest epoxy resin is prepared by the reaction of bisphenol A (BPA) (80-05-7)
with epichlorohydrine(ECH) (106-89-8) (Scheme II). The value of n varies from 0 to 25.
This determines the end-use applications of the resin.
Fig.3.3 Scheme II
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Applications for epoxy resins are extensive: adhesives, bonding, construction
materials (flooring, paving, and aggregates), composites, laminates, coatings, molding, and
textile finishing. They have recently found uses in the air- and spacecraft industries.
A)EPOXIDATION :
There are three important methods of producing epoxides. First is catalytic
epoxidation. Here the oxidation of olefins is carried out by directly oxidizing them in the
vapor phase in the presence of a catalyst such as silver Second is epoxidation by organic
peroxides and their esters. Unsaturated compounds such as hydrocarbon fatty acids and their
esters are epoxidized by peroxyacetic acid.Third is epoxidation by inorganic peroxides and
inorganic peroxy-acids. Sodium peroxide or tungstic acid deposited on a inert surface is used
for the epoxidation of olefins by hydrogen peroxide.
B)CHEMISTRY:
Epoxy resins are prepared by the reaction of active hydrogen-containing compounds
with epichlorohydrinfollowed by dehydro-halogenation. BisphenolA (BPA) (80-05-7), on
reaction with epichlorohydrin (ECH) (106-89-8) in the presence of caustic soda, produces
diglycidyl ether of bisphenol A (DGEBPA) (1675-54-3). Here nis nearly zero (0.2). The resin
is liquid when n < 1 and solid when n > 2.
C)Curing:
The curing of the epoxy group takes place either between the epoxide molecules
themselves or by the reaction between the epoxy group and other reactive molecules with or
without the help of the catalyst.The former is known as homopolymerization, or corrective
curing; and the latter is an addition or catalytic curing reaction. Both reactions result in
coupling as well as crosslinking (Scheme III).
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DEPT. OF EEE 18 AITS-HYD
Fig.3.4 Scheme III
Curing of DGEBPA with a diamine occurs in three stages: propagation of the linear chain,
formation of a branched structure, and crosslinking. Primary and secondary amines are
widely used to cure epoxy resins. The reaction between the oxiranegroup of the epoxy resin
with primary amines is shown in Scheme IV.
Fig.3.5 Scheme IV
Tertiary amines also are used to bring about catalytic polymerization of epoxy resin
and the mechanism given inScheme V. To suit the requirements of the end products, other
nitrogen compounds used for curing are triamines (DETA, TETA), polyamides (two)
obtained from vegetable oils, polyureas (two), polyisocyanates, dicyanamide, polyurethane,
and imidazole. Polymercaptans, polyhydric alcohols, polyphenols, novalacs, and silanes also
are usedfor epoxidations. Magnetic fields and photoinitiation also are used for
polymerization. Hydantoin-based epoxyresin (15336-81-9) is used to form DGEBPA.
Glycidyl esters of dimerfatty acids can also be produced fromvegetable oils. Curing agents
such as cyclic acid anhydrides are used. The reaction is shown in Scheme VI.
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DEPT. OF EEE 19 AITS-HYD
Fig.3.6 Scheme V
Fig.3.7 Scheme VI
DGEBPA also is produced from aliphatic diols such as butane-1,4-diol (2425-79-8),
propylene glycol(16096-30-3), hydrogenated BPA (13410-58-7), triglycidyl adduct of p-
aminophenol, hetrocyclicglycidyl amidesand imides, and triglycidylisocyanurate (2451-62-
9). Lewis acids such as boron trifluoride complexes are also usedas curing agents. Cationic
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DEPT. OF EEE 20 AITS-HYD
catalysts such as metal halides, coordination catalysts such as metal chelates
andphotoinitiation are used to bring about polymerization.
3.6. THE MANUFACTURING PROCESS
The epoxy resins can be obtained in either liquid or solid states.
a)Liquid Epoxy Resins :
In this process ECH and BPA are charged into a reactor in the ratio of 10:1. A
solution of 20-40% caustic sodais added slowly to the reaction vessel as the solution is
brought to the boiling point. The solution is kept boilinguntil 2 mol of caustic soda per mole
of BPA have been added. The solution breaks up into two layers. UnreactedECH is removed
by vacuum distillation. An inert solvent is then added to the resin and the reaction is
completedwith excess of caustic soda solution. The resin separates into brine solution, which
is thoroughly washed with waterto obtain a clear resin. The solvent is removed by vacuum
distillation.
b)Solid Epoxy Resin :
Here ECH and BPA are added to the reactor in theoretical molar ratio with a little
excess of ECH. Aqueouscaustic soda is well mixed into the system. After one hour the
reaction is complete and a taffylike mass is obtained.Phase separation is brought about by
adding an inert solvent. Brine is withdrawn and the resin solution isthoroughly washed with
water to remove traces of salts. The solid resin is obtained by removing the solvent byvacuum
distillation.
c)Modified Epoxy Resins :
Epoxy resins form adducts with vinyl, acrylic, polyester resins, phenol novolac (9003-