FIELD OF THE INVENTION
This invention generally relates to a development of formulation of an epoxy
adhesive suitable for in-situ bonding of end packets of stator core of large size
Turbogenerator. More particularly, the invention relates to an adhesive system
for in-situ gluing of the stampings of end packets of stator core of a
turbogenerator.
BACKGROUND OF THE INVENTION
Turbo generators of different variants and ratings viz. Air Cooled type TARI from
80 to 160 MW, Hydrogen Cooled type THRI of 210 MW & 250 MW and
Hydrogen/Water Cooled type THDF & THW with ratings of 500 MW & 210/235
MW, are known.
Each Generator has a stator core with end packets which comprises stampings at
both ends below the core end plate. At present, these end packets are
manufactured separately by bonding the stampings with adhesive and then
assembled during the core building. This technology requires machining of the
surface of press fingers at the press finger plates for better contact between all
the fingers including the bonded end packet. But it is difficult to get good contact
between all the fingers and the bonded end packet. As a result, the stampings of
the end packet get loosen due to vibration during operation of the electrical
machine.
In order to obtain better contact of the core stampings with the press fingers
including the bonded end packets, a process of in-situ gluing of the stampings of
the end packets during the building of the stator core is adopted.

With the increasing of size of the Turbogenerator, the core length increases and
hence the chances of the loosing of the stampings of the end packets increase
during operation of the machine. Therefore, the technology of in-situ gluing of
the stampings of end packets of the stator core of large size generator is very
essential. The stator core length of the turbogenerators varies from 4350 mm to
6650 mm and the total time required for building the complete core may be upto
30 days. Pressing with heating is applied during building the core in three stages
at a height of 1200 mm, 6450 mm and 6600 mm. The maximum time required
for building the core upto a height of first stage viz. 1200 mm is 7 days. In this
background, the process of in-situ gluing of the stampings requires a special kind
of adhesive. The present inventors during experimentations have observed that
the adhesive to be developed need to possess at least the following properties :
1. The adhesive has to be liquid so that it can be applied easily on the
stampings.
2. It should remain soft and reactive for a period of minimum 7 days when
the first intermediate e pressing with heating is done after building the
core up to the height 1200 mm approximately.
3. It should have a pot life of minimum 6 hours at ambient temperature and
humidity.
4. It should provide adequate bonding strength between the stampings.
5. As the heating of the stator core is done by hot air reaching to a
maximum temperature of 80°C, the adhesive should be curable at
temperature of not more than 80°C for 10 hours.

Unfortunately, conventional epoxy adhesives do not meet one or many of above
requirements and are not suitable for in-situ gluing of the stampings. Therefore,
a necessity arises to develop a formulation from an epoxy resin, curing hardener
accelerator which could meet the above requirements.
OBJECTS OF THE INVENTION
It is therefore an object of this invention to propose an adhesive system for in-
situ gluing of the stampings of end packets of stator core of a turbo generator
during stacking the core in the pit.
Another object of this invention is to propose an adhesive system for in-situ
gluing of the stampings of end packets of stator core of a turbogenerator, which
comprises bisphenol-A epoxy resin, anhydride hardener and tertiary amine type
accelerator having sufficient reactivity after storage of minimum 7 days time at a
given ambient temperature and humidity.
A still another object of this invention is to propose an adhesive system for in-
situ gluing of the stampings of end packets of stator core of a turbogenerator,
which enables adjustment of its pot life and reactivity according to ambient
temperature and humidity.
Yet another object of this invention is to propose an adhesive system for in-situ
gluing of the stampings of end packets of stator core of a turbogenerator, which
is enabled to cure within a maximum temperature of 80°C, and during heating of
the stator core by hot air circulation.

A further object of this invention is to propose an adhesive system for in-situ
gluing of the stampings of end packets of stator core of a turbogenerator, which
enables adjustment of its reactivity by controlling the content of tertiary amine
varying from 1.5 to 4 parts per 100 parts of resin and hardener mix.
SUMMARY OF THE INVENTION :
Accordingly, there is provided a formulation of an epoxy adhesive which consists
of mix of bisphenol-A epoxy resin and anhydride hardener. A tertiary amine is
used as a catalyst to expedite curing of the mix of resin and hardener. This
adhesive provides a strong bond between the stampings.
The epoxy resin is selected from as group of epoxy resins, an anhydride
hardener selected from a group of anhydrides and a tertiary amine catalyst
selected from a group of tertiary amines.
In accordance with a further embodiment of this invention, the formulation of
epoxy adhesive comprises by way of example 100 parts by weight of epoxy
resin, 100 parts of anhydride hardener and a tertiary amine as a catalyst. This is
merely by way of an example and should not be construed as limiting the
invention.
The hardener compound in the above composition has been selected from a
group of liquid cyclic anhydrides such as Methyltetranhydrophthalic Anhydride
(MTHPA), Methylhexanhydrophthalic Anhydride (MHHPA), Nadic methyl
anhydride (NMA) and the like, and particularly Methylhexanhydrophthalic
Anhydride (MHHPA) is preferred from the point of view room temperature
stability.

The tertiary amine compound in the above composition has been selected from a
group of tertiary amines such as Triethyl amine, Dimethylaminomethyl phenol,
Tris (dimethylaminomethyl) phenol, Benzyldimethylamine, Alpha-Methyl
Benzyldimethylamine, Triethylene Diamine etc. In the present invention,
Benzyldimethylamine is preferred.
In accordance with another embodiment of this invention there is provided a
formulation of an adhesive comprising by way of example 100 parts by weight of
bishphenol-A epoxy resin, 90-100 parts by weight of anhydride hardener and 2-4
parts by weight of tertiary amine. This is merely by way of an example and
should not be construed as limiting the invention.
In accordance with the further embodiment of this invention, the reactivity of the
adhesive can be controlled to suit the ambient conditions of temperature and
humidity by varying the quantity of the tertiary amine catalyst.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS & TABLES
Figure 1 a graphical representation of minimum temperature of cure for the
inventive adhesive system by differential scanning calorimetry
(DSC).
Figure 2 a graphical representation of curability of the adhesive system of
the invention at 80°C.
Table 1 Shows the effect of ambient temperature on the reactivity of the
adhesive system of the invention.

Table 2 Shows the effect of humidity on the reactivity of the adhesive of
the invention.
Table 3 Shows the effect of quantity of catalyst on the reactivity of the
inventive adhesive system.
Table 4 Shows the effect of storage on the flow of the adhesive system of
the invention when coated on the stampings.
Table 5 Shows the effect of storage on the tensile shear strength of the
adhesive system of the invention.
DETAILS DESCRIPTION OF THE INVENTION
In the context of present invention, experiments were conducted to confirm the
above mentioned properties of the adhesive.
PREPARATION OF ADHESIVE :
The liquid anhydride hardener is added to the epoxy resin and stirred. Then
tertiary catalyst is added to the resin-hardener mix and stirred with the help of
glass rod to mix uniformly all the constituents.
EXPERIMENTS. RESULTS & ANALYSIS :
Determination of effect of ambient temperature on the reactivity on the
adhesive:

The reactivity of the adhesive of the present invention was determined by
carrying out gel time measurement at different temperatures viz. 21°C, 40°C &
50°C. The results are shown in Table-1.
As evident from Table-1, the gel time decreases with the increase of temperature
which implies that the reactivity of the adhesive increases with the increase of
ambient temperature.
Determination of effect of humidity on the reactivity of adhesive :
The adhesive was exposed to different level of relative humilities viz. 49%, 93%
at room temperature for 24 hours. The relative humidity of 93% was obtained by
using a closed vessel filled with water. The gel times of respective samples were
determined. The results are shown in theTable-2.
As evident from Table-2, the reactivity of the adhesive increases with the
increase of humidity.
Control of reactivity of the adhesive by varying quantity of tertiary
amine accelerator:
The gel times of the adhesives containing different quantities of tertiary amine
catalyst viz. 1.5, 2 and 3 parts per 100 parts of resin-hardener mix were
determined and the results are shown in Table-3.

As evident from Table-3, the gel time of the adhesive decreases with increase of
quantity of tertiary amine catalyst. Therefore, the reactivity of the adhesive can
be appropriately controlled by varying the quantity of tertiary amine accelerator
to suit ambient conditions of humidity and temperature.
Determination of minimum temperature for curing adhesive:
The minimum temperature required for curing the adhesive was determined by
carrying out thermal analysis on the 5 milligram sample by means of Differential
Scanning Calorimetry (DSC) in a nitrogen atmosphere at a temperature rise rate
of 10°C/minute from a temperature of 40°C to 350°C. The DSC thermogram is
shown in figure 1.
The minimum temperature required for curing of the adhesive is measured by
the point of temperature where the DSC curve deviates from the base line. As
evident from Figure-1, this temperature is 65°C.
Determination of curability of the adhesive at temperature 80°C:
The degree of cure of the adhesive cured at temperature of 80°C for different
periods was determined by Differential Scanning Calorimetry (DSC). The plot of
degree of cure vs. cure times at 80°C is shown in Figure 2.
As evident from this graph, 90% curing of the adhesive is achieved after curing
at 80°C for 10 hours.

Determination of change of reactivity of the adhesive coating during
storage at ambient conditions of temperature and humidity :
The change of the reactivity of the film of the adhesive during storage at
ambient condition of temperature and humidity was determined by carrying out
the following tests :
Determination of change in the flow of adhesive coating with storage :
This test was conducted to assess the change in amount of flow of the adhesive
from coating on the stamping after its storage at room temperature and humidity
for a period up to 16 days.
Two sheets of stampings of size 10 x 10 cm2 were coated with the adhesive of
present invention. Only one side of the above sheets was coated with adhesive.
The two sheets were joined together and stored at room temperature 31°C and
relative humidity of 50-55% up to 16 days. The resin flow of the coating was
measured after their storage of 7 days and 16 days using the following curing
regimes.
A pressure of 10kg/cm2 was applied on the sheets after reaching temperature
80°C.
The results are indicated in Table 4. As evident from the Table-4, the adhesive
present invention is flowable after its storage for up to 7 days.

Determination of changes in the reactivity of the adhesive film through
measurement of tensile shear strength :
Six number of burr free strips of size 20 x 150 mm were cut from the stampings
pre-coated with magnetic steel sheet varnish to spec. No. AA27512. One side of
area 20 x 25 mm2 of each strip was coated with the adhesive of the present
invention. Two strips were taken to make a joint kept overlapping of an area of
20 x 25 mm2 as shown below :
The joint was held in position, four sets of above test specimens were prepared
and kept at room temperature 23-26°C and relative humidity of 50-55% for 2,4
days, 1&2 weeks. Three test specimens after their storage for respective periods
at room temperature were placed between the pair of steel plates in a press. A
pressure of 25 kg/cm2 was applied and cured as per curing regimes given below:
1. At RT for 10 hr. press.
2. At 80°C for 10 hr. press.
3. At RT for 24 hr. press.
4. At 80°C for 24 hr. press.
The test specimens were cooled in press condition to room temperature before
subjecting to testing for tensile shear strength on Ultimate Tensile Strength
Machine (UTM). The results are shown in the Table 5.

As evident from Table-5, there is no deterioration in the tensile shear strength of
the bond after storage of 1 week at temperature 23-26°C and relative humidity
of 50-55%. This suggests that the adhesive remains reactive even after its
storage of 7 days which is the maximum time required for building the core upto
the certain height when first stage pressing with heating is done.

WE CLAIM :
1. An adhesive system for in-situ gluing of the stampings of end packets of
stator core of a turbogenerator, comprising epoxy resin, anhydride
hardener and tertiary amine, wherein said resin is bhisphenol-A epoxy
resin having average molecular weight 345, wherein said hardener is
Methylhexahydrophthalic anhydride, and wherein said tertiary amine
selected from a group of tertiary amine is one of benzyldymethyl amine
and 2, 4, 6 - tris (dimethylaminomethylphenol).
2. An adhesive system as claimed in claim 1, which is capable of remaining
reactive after storage of at least seven days at a given humidity and
temperature .
3. An adhesive system as claimed in claim 1, which is curable upto a
temperature of 80°C obtainable by hot air circulator during heating of the
core.
4. An adhesive system as claimed in claim 1, wherein the reactivity of the
adhesive is controlled by adjusting quantity of the tertiary amine catalyst
corresponding to varying ambient condition of temperature and humidity.

5. An adhesive system for in-situ gluing of the stampings of end packets of
stator core of a turbogenerator as substantially described and illustrated
herein with reference to the accompanying drawings.

The invention relates to an adhesive system for in-situ gluing of the
stampings of end packets of stator core of a turbogenerator, comprising
epoxy resin, anhydride hardener and tertiary amine, wherein said resin is
bhisphenol-A epoxy resin having average molecular weight 345, wherein
said hardener is Methylhexahydrophthalic anhydride, and wherein said
tertiary amine selected from a group of tertiary amine is one of
benzyldymethyl amine and 2, 4, 6 - tris (dimethylaminomethylphenol).