TITLE:
A surface coating material for acoustic attenuation equipments comprising a
bi-layer structure having a base coat and a top-coat.
FIELD OF THE INVENTION:
This invention relates to a surface coating material for acoustic attenuation
equipments comprising a bi-layer structure having a base coat and a top-
coat.
BACKGROUND OF THE INVENTION:
A number of techniques are known for noise and vibration damping by
employing simple passive barrier, damping concepts, absorptive techniques
and electronic noise cancelling approaches. Absorptive techniques are
utilized to prevent or reduce air borne acoustic energy from reaching a
receiving site. The 'mass law' applies to a relatively thin, homogeneous and
single layer panel and states that the loss of energy, as it transits a barrier
over a wide frequency range, is a function of the surface density of the
barrier material and the frequency. By increasing the mass of the material
and thickness of the layer and density, the acoustic barrier for all frequencies
including those in the lower region in the spectrum could be improved.
Chinese Patent Publication Number CN203554179 U dated April, 16, 2014
discloses the construction of noise reduction structure, which includes a

motor, the motor mounts and gear box housing, in which the motor is fixed
on the motor mounts and characterized in that the gearbox casing
immobilized on the surface of the motor holder, the rotation shaft of the
motor extends into the housing and the inner box of the reducer connected
to the speed reduction gear box, the motor fixing seat tightly with the upper
surface of the housing of the gear box affixed to the first layer of a cotton
insulation. According to the technical scheme of the utility model, the
structure is capable of effectively preventing the sound reflection and the
vibration of the motor and the gear during rotation, so that the effects of
reducing noises and silencing are realized.
European Patent Publication Number EP1553693 B1 dated December 19,
2007 by S Yoshinaga et. al. discloses a technique for reducing or modifying
magnetic noise of an AC rotary electric machine. By using as a reference the
basic frequency component of multiphase AC current flowing to an armature
of a multi-phase AC rotary electric machine, higher harmonic current for
magnetic noise reduction of n-dimensional frequency is superimposed over
multi-phase AC current, thereby reducing or modifying the higher harmonic
component of the basic frequency component multiplied by n-1 among the
radius-direction magnetic excitation force generated in the radius direction of
the iron core of the AC rotary electric machine. The same group has
published another patent, i.e., United States Patent Publication Number

US7151354 B2 dated December 19, 2006 by S. Yoshinaga et. al. (Denso
Corporation) which describe techniques for reducing or altering the magnetic
noise of an AC rotary electric machine. As per this invention, a magnetic
noise reducing harmonic current of order n, whose frequency is n times the
frequency of the fundamental frequency component of a poly-phase AC
current fed to an armature of a poly-phase AC rotary electric machine, is
superimposed on the poly-phase AC current, thereby reducing or altering a
harmonic component having a frequency (n-1) times the frequency of the
fundamental frequency component and occurring due to a radial magnetic
excitation force acting radially on an iron core of the AC rotary electric
machine.
United States Patent Publication Number US6069422 A dated May 30, 2000
by B. D. Garrison et. al. (Fasco Industries, Inc) describes the design of an
electric motor design and method thereof useful to reduce noise/vibration
generated by a rotor/shaft assembly wherein the motor has a stator which
has an axial length and a rotor which has an axial length less than the axial
length of the stator such that when the rotor rotates within the motor and the
rotor is axially fixed by a stator field. The same group has another patent
publication number

USRE38977 El dated February 14, 2006 describes a method of reducing
noise/vibration caused by oscillation of an electric motor rotor by providing a ,
stator having an axial length and further by providing a rotor having an axial
length which is about between 1/8 to 2/8 inches shorter than said axial length
of said stator and also by applying an electric current to said electric motor
which generates a magnetic flux within said electric motor to cause said rotor
to rotate with a stator field.
United States Patent Publication Number US6078117 A dated June 20 2000
by Randall L Pernn et al discloses an end cap assembly, which is configured
as an electronic conducted noise filter and/or radiated field shield for
incorporation as the end cap of a motor housing case as original equipment.
As reported, the assembly includes a conductive panel having conductive
members which interconnect discrete electrical components of a filter
network array. Alternatively, various types of printed circuit boards,
wireframe assemblies, wirewrap assemblies, and lead frame assemblies
may be used. These assemblies include the conductive members which are
held in position within an electricalinsulative component member or housing
of the end cap assembly by such means as insertion of tabs into captivating
slots, insert overmolding, encapsulation, heat staking, mechanical.,
fasteners, and/or adhesives. The same inventor in another patent
publication Number US5982253 A dated November 09, 1999 discloses an

interchangeable, plug-in, in-line module for attenuating electrical noise
wherein a plurality of plug-in terminals are electrically coupled to an
attenuating circuit in the form of a filter network array including a plurality of
discrete components electrically interconnected by conductive traces or
printed circuits formed on an insulating board to facilitate a plug-in
connection to an electrical system.
Chinese Patent Publication Number CN103500576 A dated January 08,
2014 discloses a micro-porous sound-absorbing muffler plate that comprises
a plate body and sound- absorbing holes, which is characterized in that a
plurality of sound-absorbing/silencing holes, and are uniformly distributed on
the plate body, and also arranged into multiple lines or rows; the distance
between any two adjacent holes is equal. As reported, the micro-porous
muffler plate body is having a thickness of 1.0mm and silence of 2.0 mm with
the large opening side silencer hole aperture of 0.04mm and one side of the
aperture hole is 0.02 mm respectively.
United States Patent Publication Number US7423411 B2 dated September
09, 2008 by Christof Martin Sihler (General Electric Company) describes the
damping of torsional vibrations. As reported, the resistive torsional mode
damping system for a shaft of a machine includes: a sensor configured for
sensing a signal representative of torque on the shaft; a controller configured

for using the sensed signal for detecting a presence of a torsional vibration
on the shaft corresponding to a natural frequency of the shaft and for
generating control signals for damping the torsional vibration; and a damper
including a damping converter and resistor coupled to a DC output of the
damping converter, the damping converter being coupled to the machine
through a power bus and having a power rating on the order of less than or
equal to about five percent of a nominal power of the machine.
European Patent Number EP2570697 A2 March 20, 2013 by Andreas
GUNDEL et. al. (VEM Sachsenwerk GmbH) describes to a vibration
damping device for a large electric machine for different locations in the
operation and a system for damping machine vibrations with significant
amplitudes.
United States Patent Number US5485053 A dated January 16, 1996 by Amr
M. Baz et. al. describes a method for actively-controlled constrained layer
(ACLD) treatment which can be used as an effective means for damping out
vibrations and sounds from flexible structures. As per the invention, the
ACLD treatment consists of a visco-elastic damping layer which is
sandwiched between two piezo-electric layers and hence the three-layer
composite ACLD, when bonded to a surface which is subject to vibrational
and/or sound disturbances, acts as a smart constraining layer damping

treatment with built-in sensing and actuation capabilities. The sensing
capability is provided by the piezo-electric layer bonded to the surface of the
flexible structure, whereas the actuation or control capability is generated by
the other piezo-electric layer which acts as an active constraining layer.
Chinese Patent Publication Number CN202650526 U dated January 2, 2013
describes an utility model relating to a damping sound-absorbing material
structure, specifically to a damping sound-absorbing material structure
capable of reducing surface vibration of a structural member and absorbing
noise around the structural member, which can effectively reduce the
vibration of the structural member and absorb the noise around the structural
member, thereby effectively reducing overall unit noise of machinery
equipment.
Chinese Patent Publication Number CN101891990 B dated August 1, 2012
discloses a water- based sound damping coating comprising various
combinations of inorganic fillers, i.e., talc, mica, calcium carbonate, kaolin,
barium sulfate or their mixtures thereof including usage of flame retardant
materials like, aluminum hydroxide or magnesium hydroxide, zinc borate,
aluminum phosphate etc or any combinations of their mixtures thereof. The
base material includes polyurethanerwhich is part by weight, i.e., 15 to 35

parts of polymer emulsion, 10 to 20 parts of powdery flame retardant, 2 to 5
parts of auxiliary agent, 35 to 55 parts of inorganic filler, and 10 to 20 parts of
de-ionized water. The polymer emulsion is described as styrene-acrylic
emulsion, acrylic emulsion or copolymer emulsion and polyurethane
emulsion of any two of the emulsion. The coating is used as internal coating
of carriage floors, side walls and roofs of vehicles, and can play a good role
in inhibiting vibration and reducing noise.
Chinese Patent Publication Number CN101665654 A, dated March 10, 2010
discloses a spraying type polyurethane-carbamide vibration and noise
attenuation material combining various filler materials, i.e., calcium
carbonate, quartz powder, graphite, titanium dioxide, clay, vermiculite, mica,
heavy crystal stone, talc, glass flake in one or several in the formulations. As
claimed, the material has the advantages of fast curing speed, good visco-
elasticity, high intensity, excellent damping vibration attenuation noise-
reduction property and car, efficiently construct under a complex to severe
environmental condition. .
Chinese Patent Publication Number CN203497126 U, dated March 26, 2014
describes the utilization of polyurethane materials for the construction of
sound proof structures for ship crew cabins. As per the invention, the
combination of materials for sound proofing is, i.e., an adhesive layer with

thickness of 1 mm, a foam of 4 mm (with compression of 40% to the
aluminum layer), a fireproof nonwoven fabric with thickness of 0.6 mm, ;>
clamping rubber layer of 2 mm and finally a polyurethane-based two-
component noise damping paint (so- called T54 and T60 commercial trade
name) with thickness of 3 mm; all for bringing down the desired noise level
at 50 dB (A).
French Patent Publication Number W02006007273 A2 dated January 19,
2006 by Corina Bisog et. al. describes noise and vibration damping
applications using blocked polyurethane pre-polymers as coated structures
or compositions on thin metal substrates. This formulations were obtained by
reacting at least one poly-ol, a first poly- isocyanate comprising
diphenylmethane di-isocyanate, a second poiy-isocyanate having 2 viscosity
at 253C of less than about 250 mPa.s, a phenoi Mocking agent, and an
oxin.c. blocking agent, wherein the weight ratio of 'second poly-isocyanate:
first poly-isocyar.ate' ir not more than 0.55 : 1.00. As described, such
compositions are relatively low in viscosity are comparatively inexpensive to
produce, are storage-stable, and are capable of providing cured coatings
upon heating that are highly adhesive and chip-resistant.
Polyurethane formulations have also been usedHn order to improve the
tactile properties of plastic parts in the car interiors, against external
influences, such as sunlight and chemical, thermal and mechanical stress

and also to obtain particular colours and colour effects in the interiors. United
States Patent Publication Number US2O060079635 Al, dated April 13, 2006
by T. Pohl et. al. (Bayer Materialscjence Ag) describes preparations
comprise aqueous polyurethane-polyurea dispersions, hydroxy-functional,
aqueous or water-dilutable binders, polyisocyanates which may have been
hydrophilically modified, and foam stabilizers. Polyurethane resin coating
preparations (30-50 parts) have also been used as a super hydrophobic self-
cleaning purposes besides maintaining other properties like antifouling, anti-
sticking, de-noise and drag reduction, etc in the same material using poly-
tetrafluoroethylene powder as a filler (100 parts), mixed solvents of acetone
and ethyl acetate (300-500 parts) and y-aminopropyl trimethoxysilane with
refractive index of 1.421.43 (1-5 parts) respectively in the preparation
(Chinese Patent Publication Number CN101205439 A dated June 25, 2008).
One disadvantage for utilizing viscoelastic polymers is the curing
temperatures, which is normally at around 180°C that limits the applications
in case of spray coating methods, as the vibrating parts of various industrial
machineries could be associated with numerous shapes, geometry and
dimension etc, noise attenuation in numerous industrial machineries and
hence an ambient temperature curing would be preferred. Besides, several
viscoelastic polymers are not available commercially because of proprietary

nature. Hence, there is a need for developing spray-able type liquid-based
materials preferably by using common polymers as a base material with
required modifications by desired filler materials etc and importantly cures at
ambient temperature. This would let the vibrating parts of the machines with
complex shapes etc to be spray coated without employing any arrangement
for heat treatment for curing and noise attenuation could be achieved. Also,
there is a further need to develop such materials with appropriate
modifications etc, in a manner that the noise and vibration could be
maximized with structures or coated layers even with relatively low level of
thickness, i.e., in the range of 1 - 3 mm.
Electrical equipments and machineries belonging to numerous industries
related to manufacturing, automobiles, appliances, processing, operations
related to grinding, cutting, mixing, milling etc are usually composed of
various metal structures or sheets etc. Such metallic parts (particularly
sheets etc) are indeed a good source of acoustic vibrations and hence often
pose significant noise problems while in operation.
As would be explained in detail herein below, the present invention provides
application of coating materials with specified formulation on various
vibrating and noise generating parts of electrical machines for the purpose of
creating acoustic attenuation of the machines. In this context, this invention

would provide noise attenuation of electrical machineries and AC motors in
specific by utilizing costing materials those applied on the surfaces of the
vibrating parts or objects of the identified machine by maintain a so-called bi-
layer structure within a thickness profile in the range of 1 -3 mm in such
coated layer. Besides, the purpose of noise and vibration damping, the same
coated structure could also additionally serve as environmental protection of
corrosion and aesthetics as well. These and other advantages of the
invention, i.e., choice coating material with specific formulation with a given
bi-layer structure within the specified thickness profile of the coated layer etc
would be apparent from the discussion and descriptions in the subsequent
sections.
OBJECTS OF THE INVENTION:
An object of the present invention is to propose a surface coating material for
acoustic attenuation equipments comprising a bi-layer structure having a
base coat and a top-coat.
Another object of the invention is to utilize various coating materials with
specified formulations and specified structure in the coated layer for the
purpose of acoustic attenuation.

Still another object of the invention is to apply the coating materials in the
form of surface coating on various identified areas or parts of electrical
machines, electrical motors and allied machineries etc for the purpose of
noise reduction of the electrical machine.
Further object of the invention is to apply surface coating by maintaining a
so-called 'bi- layer structure' using one mate rial as a 'base coat' and another
material as a 'top coat' respectively with specified formulation/s and
thickness profile in each coated layer.
BRIEF DESCRIPTION OF THE INVENTION:
According to this invention there is provided a surface coating material for
acoustic attenuation equipments comprising a bi-layer structure having a
base coat and a top-coat. In accordance with this invention there is also
provided a method of applying the coating comprises of: coating the base
coat by spraying on the surfaces; subjecting the coated surface to the step of
curing at ambient temperature, pressure and humidity; applying the top coat
by spraying to the preferable thickness and subjecting the bilayer structure to
the step of curing.

DETAILED DESCRIPTION OF THE INVENTION:
The inventions described the process for the preparation of spray-able type
filler- modified epoxy resin hardener based and polyurethane resin-hardener
based composite materials for the purpose of acoustic-attenuation.
As per the present invention, specific material system with defined
formulations, that could be sandwiched to one another, in the form of surface
coating on various vibrating parts or noise generating parts or metallic
casings etc in various electrical machines and AC motors in specific and
allied electrical machineries and noise reduction of the machine could be
achieved. The coating materials are to be applied by maintaining specific
combinations of so-called 'bi-layer structure' in the coated layer.
Such bi-layer structure could be generated on ;he internal or external
surfaces of the vibrating parts or noise generating parts of the electrical
machine, by identifying one coating material as a 'base coat' and another
coating material as the 'top coat' respectively. The thickness of such 'bi-layer
structure' could preferably be maintained in the range of 1 - 3 mm, since the
same coating on the surface could also serve as aesthetics, as well as, a
protective coating for environmental corrosion, erosion purposes etc besides
the noise reduction.

The following steps describe the process for generating the surface coating
on metallic casing, or noise generating parts, or vibrating objects by
maintaining a specific 'bi-layer structure' of the identified electrical machine
or AC motor:
• Preparation of 'base coat' by mixing commercial grades of melamine powder
and liquid epoxy resin in specified formulation in a suitable container by
using a bail mill machine for a period of 4 - 8 hours and then mixing and
dispersing one hardener material for a period of another 15 - 30 minutes that
produces the 'base coat' preparation
• Preparation of 'top coat' by mixing commercial grades of barium sulphate
powder and the liquid polyurethane resin in specified formulation in a
suitable container by using a ball mill machine for a period of 4 - 8 hours and
then a mixing and dispersing one hardener material for a period of another
15 - 30 minutes that produces the 'top coat' preparation
• Coating the 'base coat' preparation by spraying or by similar procedure/s on
surfaces (either internal and external surfaces or both) of metallic casing and
or other vibrating parts or noise generating parts of the AC motor and then
curing at ambient temperature, pressure, humidity etc and by maintaining a
preferable thickness profile in the coated layer of the 'base coat'.

• Coating the 'top coat' preparation by spraying or by following similar
procedure/s on the surface of the 'base coat' that is already been cured and
thereby curing the 'top-coat' layer at ambient temperature, pressure,
humidity etc and by maintaining a preferable thickness profile of the coated
layer of the 'top coat'
• Generating a specified 'bi-layer structure' on the surface (either internal or
external or both) of metallic casing and other vibrating part/s of electrical
machine or AC motor with a preferable thickness profile in the coated layer
• Evaluating the acoustic attenuation characteristics of the generated 'bi-layer
structure' in an electrical machine, i.e., 'AC motor
• Establishing the complete process of surface coating using specific
combinations of 'base coat' and 'top coat' coating materials respectively
thereby forming a 'bi-layer structure' with specified thickness profile in the
coated layer for the purpose of acoustic attenuation of electrical machines or
AC motors and allied areas.
The present invention refers to application of coating materials with specific
formulation/s to the surfaces of various vibrating or noise generating parts in

electrical machines and particularly in AC motors and or allied areas for the
purpose of acoustic attenuation of the target machine.
In order to achieve this accomplishment, one coating material with defined
formulation, known as 'base coat' is to be prepared first, which is then to be
applied on the surfaces (internal or external or both) of the identified
vibrating part/s or noise generating part/s in a given electrical machine or an
AC motor. After applying, the coating material is to be cured after which the
so-called 'base coat' is generated on the surfaces of the vibrating or noise
generating part of the machine.
The aforesaid 'base coat, material is liquid-based and spray-able type which
cures at ambient temperature and humidity etc. and is prepared in two steps.
In the first step, commercial grades of epoxy resin (di-glycidyl ether bis-
phenol A) is mixed with melamine (C3H6N6) powder by maintaining specific
weight ratio in the range of 50 - 60 weight percentage by using a ball mill
machine or a suitable mixing or milling machine for a period of 4 - 8 hours in
order to get well dispersed homogenized melamine- modified epoxy resin
based formulation/s. In the second step, a hardener material, i.e., tri-
ethylene-tetramine is mixed to the resultant melamine-modified epoxy resin
formulation/s, which is then homogenized for a period of another 15-30

minutes by using a ball mill machine or similar mixing or milling machine that
results the spray-able type liquid coating material known as the 'base coat'.
The thus-derived 'base coat' is applied on the surface of the metallic casing
(inner area or outer area both) of any identified AC motor by spraying by
maintaining a nozzle pressure in the range of 70 - 80 psi by which the base
coat layer develops. The thickness of the base-coat layer in the pre-cured
stage (green stage) could be developed by increasing the number of
spraying pass in the object, however could be restricted in the range of 0.5 -
2.0 mm out of a total thickness of about 3 mm (combining both base-coat
and top-coat, which would be discussed in the subsequent sections), as
similar coating with specified thickness profile could also serve as protective
coating for erosion, corrosion and aesthetics purposes of the casing of the
equipment as well. The base- coat, irrespective of thickness profile cures at
ambient temperature, pressure, humidity conditions etc. Besides, generating
higher level of thickness, the spraying process could also be repeated in
order to eliminate surface voids, cracks or defects etc that might often
generate post-curing of the base-coat layer.

After the base-coat with a specific thickness profile has been cured, the next
step is to prepare the coating material for the 'top coat' with specified
formulation, which is to be applied on the cured surface of the base-coat by
maintaining specific thickness profile of the top coat layer.
The- aforesaid 'top coat' material with specified formulation is prepared by
mixing commercial grades of polyurethane resin (acrylic polyol) material with
barium sulphate (BaS04) powder by maintaining a weight ratio in the range of
50 - 60 weight percentage and by homogenizing by a ball mill machine or
similar mixing or milling machine using ceramic or metal balls as mixing
media for a period of 4 - 8 hours in order to get well dispersed barium
sulphate modified polyurethane resin formulation/s. The resultant
formulation/s is then mixed with a hardener (aliphatic iso-cyanate) material
for a period of 15 - BO minutes by using the same ball mill machine or
similar mixing or milling machine that results the coating material for the said
'top coat'.
The thus-derived 'top coat' material is applied on the cured surface of the
previously applied 'base-coat' by spraying thereby maintaining a
corresponding pressure in the range of 70 - 80 psi. The thickness of the top-
coat layer in the pre-cured stage (green stage) could be developed, in a

similar manner like the previous base-coat, i.e., by increasing the number of
spraying pass in the object, however could be restricted in the range of 0.5 -
1.5 mm out of a total thickness of about 3 mm (combining both base-coat
and top-coat), as the similar thickness profile could also serve as protective
coating for erosion, corrosion and aesthetics purposes as well. The top-coat,
irrespective of thickness profile also cures at ambient temperature, pressure,
humidity conditions etc. Besides, generating higher level of thickness, the
spraying process could also be repeated in order to eliminate surface voids,
cracks or defects etc that might often generate post-curing of the top-coat
layer.
The combination of such sandwiched 'base-coat' and 'top-coat' on the
surface of the vibrating objects or parts in an electrical machine and more
specifically to casings of AC motors represent 'bi-layer structure'. Various 'bi-
layer structure/s' could hence be generated by selecting various coating
materials belonging to 'base-coat' and 'top-coat' respectively by varying the
amount of melamine powder material in the 'base-coat' and 'barium sulphate
material in the 'top coat' respectively in the specified range of 50 - 60 weight
percentage in their respective parent resins and by keeping the hardener
amount more or less unchanged.

The thus-generated 'bi-layer coated structures' additionally could serve as
protective coating against environmental corrosion, erosion etc for the metal
casings besides aesthetics as well.
The noise attenuation for each 'bi-layer structure' that has been generated
by coating on the surfaces of noise generating parts or vibrating parts of the
identified electrical machine or specifically to casing of AC motors, could be
evaluated by comparing between the noise reading of the machine without
any coated material in the structure (blank) to that of noise reading of :he
machine with bi-layer coated structure and the effectiveness of noise
attenuation involving such 'bi-layer structure' in the form of surface coating in
the vibrating part/s or noise generating part/s or specifically in casings of AC
motors could be realized.
In order to accomplish the noise attenuation characteristics of various 'bi-
layer structures' using numerous combinations of 'base-coat' and 'top-coat'
coating materials, both the internal and external surfaces of metallic casing
of one identified AC motor, was coated with specified 'base-coat' and 'top
coat' coating materials and various 'bi-layer composite structures' were
generated that had a total thickness in the range of 1 - 3 mm, combining
both base-coat and top-coat together. The noise attenuation characteristics
of the AC motor could be realized by comparing the noise level of the

machine in blank condition (without any coating on the metallic casing
surfaces) to that of the noise level after generating the 'bi-layer structure' in
the metallic casing, under identical conditions.
The process could be more realized by citing appropriate examples which
are brought out in the following sections.
Example 1:
The coating material for the 'base coat' was prepared as per following
procedure in this example:
80 volume parts of epoxy resin (diglycidyl ether bis-phenol A) was mixed
with 60 weight: percentage of melamine (C3H6,N5) powder in a plastic
container and allowed to be milled for a period of about 4 hours in a ball mill
machine using cera.mic balls as the mixing or grinding media and by
maintaining a rpm of 80, after which a 'melamine-modified epoxy resin'
resulted. To this 'filler-mixed epoxy resin', 20 volume parts of hardener
(triethylene-tetramine) was added and the mixing operation was continued
for a period another 20 minutes, which results a spray-able type coating
material. This resultant coating material is known as the 'base coat' in this
example, which possesses a MOE (moduius or elasticity) value of 2.34 ± 0.1
GPa.

The derived 'base coat' is applied on the internal surface of the metallic
casing (which is made of stainless steel with a thickness of 10 ± 0.5 mm)of
one identified AC motor (2100 kW, 6.6 kV) by spraying thereby maintaining a
corresponding pressure in the range of about 80 psi. The thickness of the
base coat layer in the pre-cured stage (green stage) was developed to about
0.9 -1.0 mm by making about 12 -15 numbers of spraying pass in the
surface. The base-coat cured at ambient temperature, pressure, humidity,
conditions etc. within a period of 8 - 20 hours. Repetition of the spraying
process also eliminated certain surface voids etc and a defect-free and
continuous structure of the base-coat was obtained after curing with uniform
thickness of 0.8 + 0.1 mm in the coated layer.
In order to prepare the 'top coat' in this example, 80 volume parts of
polyurethane resin (acrylic polyol) was mixed and milled with 60 weight
percentage of barium sulphate powder material in a plastic container for a
period of about 4 hours in a standard a 'ball mill machine' using ceramic balls
as mixing media with a counter rpm of about 80 in the machine, after which a
'barium sulphate modified polyurethane resin' resulted. To this preparation,
about 20 volume parts of hardener (aliphatic iso-cyanate) was added and
mixing operation was continued for another period of 20 minutes that results
the 'top coat' coating material in this example. Such a 'top coat' material has
a MOE (modulus of elasticity) value of 5.71 ± 0.1 GPa.

The 'top coat' was then sprayed on the surface of the cured layer of the base
coat in the internal surface of the metallic casing of the AC motor by using a
spray gun and by maintaining the pressure level of about 80 psi and thereby
cured under natural conditions. The total thickness after the curing resulted
to about 1.3 ±0.2 mm.
The AC motor with its internal surface metallic casing coated with the afore-
described bi-layer coated structure having thickness profile in the range of
1.3 + 0.2 mm with was then operated and the 'A-weighted noise levels' of
both 'LAeq(dB)' and 'LAFmax(dB)' were recorded after initial stability of the
noise level by following IEC 61672-2:2006 noise measurement norms. All
the noise levels, i.e., 'A-weighted noise levels' of both 'LAeq(dB)' and
'LAFmax(dB)' were also recorded before applying the coating material in bi-
layer structure in the metallic casing of the same AC motor. The differential
reading in 'LAeq(dB)' noise level provides the reduction in noise level due to
application of coating materials in the metallic casing of the identified AC
motor in specific bi-layer structure using specified coating materials as 'base
coat' and 'top coat' respectively. The reduction in noise level was in the
range of 2 - 3 dB from an initial noise level, LAeq(dB) of 88 dB.

Example 2:
In this example, a similar kind of bi-layer structure was generated by
following the same procedure to that of the Example 1, however the
structure was generated outside or external surface of the metallic
casing of an identical AC motor.
The thickness profile in the 'base coat' after curing was in the range of
0.85 ± 0.1 mm and the final combined thickness after the top coat was
1.35 ±0.2 mm.
The noise level of the AC motor i.e., 'A-weighted noise levels' of both
'LAeq(dB)' and 'LAFmax(dB)' were recorded with its metallic casing
(external surface) coated afore-described bi-layer coated structure
having thickness profile in the range of 1.35 ± 0.2 mm by following IEC
61672-2:2006 noise measurement norms. All the noise levels, i.e., 'A-
weighted noise levels' of both 'LAeq(dB)' and 'LAFmamax(dB)' were
also recorded before applying the coating material in bilayer structure in
the metallic casing (external surface) of the same AC motor. The
differential reading in 'LAeq(dB)' noise level provides the reduction in

noise level due to application of coating materials in the metallic casing
(external surface) of the identified AC motor in specific bi-layer structure
using specified coating materials as 'base coat' and 'top coat'
respectively. The reduction in noise level of the AC motor was in the
range of 2-3 dB from an initial noise level, LAeq(dB) of 88 dB.

WE CLAIM:
1. A surface coating material for acoustic attenuation equipments comprising:
a bilayer structure having a base-coat and a top-coat.
2. The surface coating material as claimed in claim 1 wherein the said base
coat comprises of mixture of melamine powder and liquid epoxy resin and a
hardener material dispersed there in.
3. The surface coating material as claimed in claim 1, wherein the said "top-
coat" comprises of mixture of barium sulphate powder and liquid
polyurethane resin and a hardener material dispersing there in.
4. The surface coating material as claimed in claim 1, wherein the thickness of
said bilayer structure is in range of 1 to 3 mm.
5. A method of applying the coating comprises of:
coating the base coat by spraying on the surfaces;
subjecting the coated surface to the step of curing at ambient temperature,
pressure and humidity;

applying the top coat by spraying to the preferable thickness and
subjecting the bilayer structure to the step of curing.
6. The method as claimed in claim 5, wherein the said base coat is prepared by
mixing epoxy resin(di-glycidyl ether bis-phenol A) with melamine (C3H6N6)
powder by maintaining a weight ratio of 50-60 wejght percentage using ball
mill machine for 4 to 8 hrs, hardener material i.e. tri-ethylene-tetramine is
mixed to produce the spray-able type liquid coating material.
7. The method as claimed in claim 5, wherein the said top coat is prepared by
mixing polyurethane resin(acrylic polyol) material with barium sulphate
powder in a weight ratio of 50-60 weight percentage, homogenized by a ball
mill machine for 4-8 hrs,
adding a hardener(aliphatic iso-cyanate) material for a period of 15-30
minutes by using ball mill machine.