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					Finishing technology with finesse



P
         owder coating has been called the coating of the
         future. Powder Coating is basically a dry
         finishing technique which is mainly used in
various sectors like industrial, marine, and consumer
goods. The Powder Coating process takes place by
heating a metal and spraying the powder on the hot
substrates     in     order     to     make      the      metal     become       even      harder.


Powder coating is a type of coating that is applied as a free-flowing, dry powder. The main
difference between a conventional liquid paint and a powder coating is that the powder coating
does not require a solvent to keep the binder and filler parts in a liquid suspension form. The
coating is typically applied electrostatically and is then cured under heat to allow it to flow and
form a "skin." The powder may be a thermoplastic or a thermoset polymer. It is usually used to
create a hard finish that is tougher than conventional paint. Powder coating is mainly used for
coating of metals, such as "white ware", aluminum extrusions, and automobile and motorcycle
parts. Newer technologies allow other materials, such as MDF (medium-density fiberboard); to
be powder coated using different methods.


Powder Coatings are popularly used for various decorative, functional and protective purposes.
Metal furniture, baby strollers, office supplies, lighting fixtures and every day home appliances
are some of the areas where powder coatings have been used.
In powder coating, the powdered paint may be applied by either of two techniques:
    The item is lowered into a fluidised bed of the powder, which may or may not be
    electrostatically charged, or
    The powdered paint is electrostatically charged and sprayed onto the part.


The part is then placed in an oven and the powder particles melt and coalesce to form a
continuous film.
There are two main types of powder available to the surface finisher:
(1) Thermoplastic powders that will remelt when heated, and
(2) Thermosetting powders that will not remelt upon reheating.


During the curing process (in the oven) a chemical cross-linking reaction is triggered at the
curing temperature and it is this chemical reaction which gives the powder coating many of its
desirable properties. Powder coatings exhibits lustrous shine, outstanding colours and a
durable, long-lasting finish. The powder which is used to coat metal is made up of thermoplastic
/thermoset polymer which make the coated substrate hard and tough in the end.


Powder Coating Process
Powder coating is a dry finishing process, using
finely ground particles of pigment and resin that
are generally electrostatically charged and
sprayed onto electrically grounded parts. The
charged powder particles adhere to the parts
and are held there until melted and fused into a
smooth coating in a curing oven. Before coating,
the parts to be coated are first pretreated
similarly to conventional liquid coated parts. The
pretreatment process is normally conducted in series with the coating and curing operations.


There is essentially two common ways of applying powder coating:
        By electrostatic spray and
        By fluidized bed powder coating.
There are several other processes that have been developed, but they are far less used. These
include flame spraying, spraying with a plasma gun, airless hot spray, and coating by
electrophhoretic deposition.
An overview of the technology used in the powder coating industry is:
Electrostatic Spray
Fluidized Bed
Electrostatic Fluidized bed
Powder Curing


Electrostatic spray process
The powder is applied with an electrostatic spray gun to a part that is at
earth (or ground) potential. Before the powder is sent to the gun it is
fluidised to separate the individual grains of powder and to improve the
electrostatic charge that can be applied to the powder so that the powder
flows more easily to the gun.


Because the powder particles are electrostatically charged, the powder wraps around to the
back of the part as it passes by towards the air off take system. By collecting the powder, which
passes by the job, and filtering it, the efficiency of the process can be increased to 95% material
usage.


The powder will remain attached to the part as long as some of the electrostatic charge remains
on the powder. To obtain the final solid, tough, abrasion resistant coating the powder coated
items are placed in an oven and heated to temperatures that range from 160° to 210 °celsius
(depending on the powder).
Under the influence of heat a thermosetting powder goes through 4 stages to full cure.
                      Melt


                      Flow


                      Gel


                      Cure
The final coating is continuous and will vary from high gloss to flat matt depending on the design
of the powder by the supplier.


Powder coating guns
There are at least three types of electrostatic guns in use:
Corona Charging:    Corona charging guns where electric power is
used to generate the electrostatic charge. Corona guns are either
internal or external charging.
Tribo Charging:   Tribo charging guns where the electrostatic charge
is generated by friction between the powder and the gun barrel.
Bell Charging:   Bell charging guns where the powder is charged by
being "flung" from the perimeter of the "bell"
Not all powder is applied using guns. One system makes use of electrostatic tunnels.


Electrostatic spray powder coating uses a powder-air mixture from a small fluidized bed in a
powder feed hopper. In some cases, the feed hoppers vibrate to
help prevent clogging or clumping of powders prior to entry into the
transport lines. The powder is supplied by a hose to the spray gun,
which has a charged electrode in the nozzle fed by a high voltage
dc power.


Electrostatic powder spray guns direct the flow of powder; control
the deposition rate; control the pattern size, shape, and density of
the spray; and charge the powder being sprayed .The spray guns
can be manual (hand-held) or automatic, fixed or reciprocating, and
mounted on one or both sides of a conveyorized spray booth.
Electrostatic spray powder coating operations use collectors to
reclaim over-spray. This reclaimed powder is then reused, adding
significantly to the powder coating's high transfer efficiency.


Fluidized bed
The fluidized bed coating process is a simple dipping process that can be either conventional or
electrostatic. In the convention fluidized bed process, the fluidized bed is a tank with a porous
bottom plate. The plenum below the porous plate supplies low pressure air uniformly across the
plate. The rising air surrounds and suspends the finely divided plastic powder particles, so the
powder-air mixture resembles a boiling liquid. Products that are preheated above the melt
temperatures of the powder are dipped in the fluidized bed, where the powder melts and fuses
into a continuous coating. A high transfer efficiency results from little drag out and no dripping.
The fluidized bed powder coating method is used to apply heavy coats in one dip, 3 - 10 mils-1
mils = 0.0254 millimeters (75 - 250 µm), uniformly to complex shaped products. It is possible to
build a film thickness of 100 mils (2500 µm) using higher preheat temperatures and multiple
dips.


Electrostatic fluidized bed
An electrostatic fluidized bed is essentially a fluidized bed with a high voltage dc grid installed
above the porous plate to charge the finely divided particles. Once charged, the particles are
repelled by the grid, and they repel each other, forming a cloud of powder above the grid. These
electrostatically charged particles are attracted to and coat products that are at ground potential.
Film thicknesses are similar to what can be achieved in the electrostatic spray process.


Electrostatic fluidized bed application uses the same fluidizing technique and the conventional
fluidized bed dip process but with much less powder depth in the bed. An electrostatic charging
media is placed inside the bed so that the powder material becomes charged as the fluidizing
air lifts it up. Charged particles of powder move upward and form a cloud of charged powder
above the fluid bed. When a grounded part is passed through the charged cloud the particles
will be attracted to its surface. The parts are not preheated as they are for the conventional
fluidized bed dip process.


The advantages of electrostatic fluidized bed coating is that preheating of parts is generally not
necessary and small products, such as electrical components, can be coated uniformly and
quickly. The disadvantages are that the product size is limited and inside corners have low film
thickness owing to the well known Faraday cage
effect.


Powder Curing
When a thermoset powder is exposed to
elevated temperature, it begins to melt, flows
out, and then chemically reacts to form a higher
molecular weight polymer in a network-like structure. This cure process, called cross linking,
requires a certain degree of temperature for a certain length of time in order to reach full cure
and establish the full film properties for which the material was designed. Normally the powders
cure at 200°C (390°F) in 10 minutes. The curing schedule could vary according to the
manufacturer's specifications.


The application of energy to the product to be cured can be accomplished by convection cure
ovens or infrared cure ovens.


Thermoplastic powders require heat only to fuse the powder together into a continuous film.
However, thermosetting powders often require additional heat to cure the film on the product.
There are four basic methods normally used in the curing of powder coated parts: convection,
infrared, a combination of the two, and ultraviolet (UV) curing.


Convection ovens can be either gas or electric. Hot air is circulated around the powder coated
parts, and the parts attain the temperature within the oven. UV curing is commonly used with
heat sensitive substrates. Specifically formulated UV powders flow at very low temperatures
(121°C) and can be cured via UV radiation in a matter of seconds.


Infrared ovens, using either gas or electricity as their energy source, emit radiation in the IR
wavelength. The radiated energy is absorbed by the powder and the substrate immediately
below the powder, so the entire part need not be heated to the cure temperature. This allows a
relatively rapid heat rise causing the powder to flow and cure when exposed for a sufficient time.
Combination ovens generally use IR as the first zone to melt the powder quickly. This process is
termed near infrared (NIR) cure, and powders are formulated specifically to take advantage of
this process. The part then progresses into a second zone, which is a convection oven.


Formulation
As with any coating, formulation variables are critical to the processing and performance
characteristics. The powder coating formulation is much like a liquid coating formulation except
that most of the components are in solid, melt process able form. The main raw material
components used in powder coatings are described below.
Resins are the key component of powder coatings; the range of resins used is increasing
steadily in an attempt to meet the more demanding needs of new market sectors.
Curing agents are used according to the type of resin system employed and the final properties
required of the coating.
Accelerators are used to increase the cure reaction
rate.
Pigments are generally solid particulate materials
such as titanium dioxide or carbon black.
Fillers are used to reduce the cost of the coating
formulation and / or to improve specific properties
such as flow, surface texture, lubricity, etc. Common
fillers are barytes, calcite, mica, talc, whiting, and
wollastonite.
Extenders such as aluminum silicate, are used to provide opacity and act as a filler.
Degassing agents are particularly important in low-bake systems. They are used to eliminate /
dissipate gas bubbles that may cause film porosity and embrittlement or poor finish.
Dry Flow agents improve the free flow of powders within the production delivery systems.
Flow agents enhance film properties and minimize / eliminate surface defects by improving the
flow of the molten coating. Examples of flow agents include polyacrylates, silicones, surfactants,
and fluorinated alkyl esters.
Matting agents are used to reduce the gloss of the cured film.
Texturing agents are used to control / enhance the gloss level of cured films.
Rheological additives provide viscosity control to molten coatings for improved edge coverage
or for textured surface effects.
Waxes are added to the formulation to provide slip, hardness, scratch and mar resistance, and
to act as free flow powders and anti-bridging agents in processing.


Uses of Powder Coating Equipments
Powder coating equipment is used to spray resin powder onto the surface of a pre-heated
component, where the powder fuses and cures producing a protective coating. Powder coating
equipment is available in complete systems, or as individual components or sub-systems.
Coating outfits, kits or packages are small, portable machines or systems with all of the
components required for the coating process.
Powder coating equipments include electrostatic coating equipment and fluidized bed systems.
Electrostatic coating charges paint droplets or coating powder particles and then sprays them
toward a part with an opposite and attractive electric charge. Since the charged powders in the
spray stream are attracted toward the part, the electrostatic process help minimize overspray
and waste. In fluidized bed systems, air is blown up through a porous bed or screen to suspend
coating particles thereby forming a fluidized bed. Hot parts are inserted into the fluidized bed
allowing the powder coating particles to stick onto the exposed surfaces. Coating powders in a
fluidized bed can also be charged for the application of thicker coatings.


Powder coating equipment provides different options for electrostatic or charging technology.
The primary force for material transport in corona powder coating systems is an electric field
established between the charged powder cloud and the part. Corona powder coating guns use
a high-voltage generator or power supply to disassociate air and produce a stream of negative
or positive ions, which in turn imparts a like charge to the powder cloud. The potentials of the
charged cloud and gun electrode combine to generate an electric field with the opposite pole
(the part). Charged powder particles at the gun seek the lower potential of the opposite pole (the
part).


With tribostatic powder coating equipment, a tribo gun imparts a charge to the powder strictly by
physical contact and movement between the powder and a surface capable of donating or
receiving electrons. Tribo charging is the process of creating a static electric charge on powder
particles by friction against a non-conductive material. All powder guiding parts in the gun are
lined with insulating materials. The gun has a friction body with a complex pattern of tabulators
through which the powder passes to maximize mixing and contact with the surface providing
enhanced charging. The velocity at which powder is liberated from the gun, and not the field
potential between the part and powder, controls the powder transport.
Various other equipments used in powder coating process are:

    •    Conveyorized Powder Baking Oven
    •    Camel Back Oven
    •    Electrical Oven
    •    Conveyor Drive Unit
    •    Main Control Panel
    •    Diesel / Gas Fired Oven
    •    Powder Coating Booth
    •    Powder Coating Gun
    •    Baking Oven Trolley
    •   Conveyorized Powder Coating Booth
    •   Conveyorized Floor Mounted Tunnel.

    Project Economics (Capacity 1,95,000 Sq.ft./yr.)

               S.No             Particulars                                     Estimated Cost
                                                                                (Rs. lakhs)
               1.        Land (500 sq. m.)                                        2.50
                                                                                 
               2.        Building (300 sq.m.)                                   10.50 
               3.        Plant & Machinery                                        6.00
                              a. Electrostatic powder spray equipment            
                                   comprising of:-
                                (i)    High voltage
                                (ii)   Powder hopper
                                (iii)  Spray gun with cables
                                (iv)   Control console
                                (v)    Powder output 30kg/hr. output
                                       voltage 90KV, DC negative variable
                                       nominal

                              b. Powder spray chamber with cyclone and
                                 bag filter recovery type of unit, fluidoline
                                 mini, recovery system cyclone type
                                 percentage of recovery 95% of over spray
                                 powder

                              c. Powder curing oven with temp. controller
                                 control panel, blower and trolley, size
                                 3’x3’x6’

                              d. Air compressor to provide moisture free,
                                  oil free, air with all accessories 
                                   
               4.        Misc. Fixed Assets                                     2.00 
               5.        Pre-operative Expenses                                 1.00 
               6.        Margin Money for working Capital                       2.00 
               7.        Contingencies                                          1.00 
                         Total Cost of Project                                  25.00

 

Market Trend

Powder coating is the fastest growing segment of the coating industry. Powder coating sales
have grown at the expense of other types of coatings, and manufacturers are converting from
liquid coating to powder coating. The conversion from liquid to powder is happening for several
reasons. First, it is environmentally friendly. Powder coating causes no air emissions and
produces no hazardous waste. Second, it is less expensive. Applying powder coating costs one
tenth of the cost of the application of liquid coating.


New entrepreneur should initially target the custom refinishing of automobile, and electronics
parts. Why? First, these individuals demand a high quality finish. Second, these individuals are
willing to spend more money to get a high quality finish. Third, these individuals know that a
powder coating finish is superior to a liquid coating finish in performance and look.


The list of current markets for powder coating is extensive as manufacturers of items ranging
from automobiles to toys, mechanical pencils, and appliances have turned to powder coating to
avoid using solvent based paints. A summary of the major markets and products where powder
coatings have the greatest commercial success may be found in the following sections.


                        Automotive Industry
                        Appliances Industry
                        Outdoor Furniture, Lawn and Garden Products
                        Architectural and Building / Construction Industry
                        Electrical / Electronic Industry
                        Non-Metallic Products


Automotive Industry

The automotive industry is increasing its use of powder coatings for economic, quality, and
environmental reasons. Powder is being used in the automotive industry for the exterior bond
intermediate coat known as a "primer surfacer", as well as for finishing of underhood
components. Powder is being used for the exterior body intermediate coat on select models of
vehicles.


Clear powder coatings, as an alternative to solvent borne clear coats, are being used for
automotive applications such as finishing wheels. Clear coats are also now being used over
automotive exterior basecoats by some high end manufacturers.


Parts that require extra protection as well as a decorative finish are increasingly being powder
coated. Wheels, hubcaps, door handles, radiators, decorative trim, bumpers, shock absorbers,
mirror frames, oil filters, engine blocks, battery trays, and coil springs are some of the many
automotive products being powder coated.
Appliances Industry


The appliance industry is the largest single market sector for thermosetting powders; it accounts
for about 1/3 of all powder sales. The appliance industry benefits from the use of powder
coating in applications requiring the combination of appearance and resistance to abrasion,
chemicals, temperature cycling, and abusive ware.


The appliance industry makes use of powder coating on front and side panels of ranges and
refrigerators, washer tops and lids, dryer drums, air-conditioner cabinets, water heaters,
dishwasher racks, freezer cabinets, and cavities of microwave ovens. Powder coatings has also
replaced porcelain enamel on many washer and dryer parts


Outdoor Furniture, Lawn and Garden Products


Outdoor furniture, farm implements, and lawn and garden equipment are also major markets for
powder coatings. The excellent durability and corrosion resistance of powder coatings make
them ideal for products that will see rough use outdoors.


The farm segment has powder coated tractors and agricultural equipment.

Golf clubs and carts, bicycles, and exercise equipment represent applications in the recreation
market segment. Homeowners have powder coatings on lawn mowers, barbecue grill, patio
furniture, garden tools, bird feeders and pet cages etc.


Architectural and Building / Construction

The architectural and building market is growing for powder coated products because of the
excellent durability of powder coatings and the variety of finishes and colors that are available.
In India, powder coating industry is working diligently to expand powder coating use.


The recent advances in polyester-TGIC and fluoropolymer powders have enabled powder
coatings to compete with liquid architectural coatings in durability, weather ability, and
resistance to fading. These are being used on outdoor stadium seating and other exterior
applications that were previously susceptible to degradation from ultraviolet rays.
Aluminum extrusions used on frames for windows and doors are powder coated as are building
facades, fixtures, and modular furniture. Many highway and building projects use powder
coating on products such as light poles, guard rails, signs, posts, and fencing.


Light poles, lawn furniture, shopping carts and shelving often benefit from powder with
fluorocarbon (polytetrafluoroethylene) additives for increased resistance to abrasion. Clear coat
powders are often used as an external protective layer on many brass products such as door
knobs, hinges, railing, lamps, and plumbing fixtures. There are also powders that can substitute
for chrome and brass plating in certain applications.


Electrical / Electronic

Powder coatings are used for exterior housings in the electrical and
electronic industries. However, they are also being evaluated for unique,
functional applications. Powder coating of copper and aluminum magnet
wire significantly reduces environmental problems in the wire industry, and
the toughness and durability of powder coatings in applications such as
transformer and motor windings provide an added bonus.


Makers of electronic components sometimes use electroconductive and electrodissipative
powder coatings. These coatings provide electrostatic discharge protection of the components,
critical during the manufacture, testing, and transport of electronic goods.


Non-Metal Products

Originally powder coatings were applied as metal finishing coatings only. However, with the
development of powder that can be applied and cured at low temperatures, the market has
opened to heat sensitive substrates such as plastics and wood. Radiation curing (UV or electron
beam) allows the curing of powder on heat sensitive substrates by reducing the curing
temperature to below 121°C. Ongoing development has been devoted to formulating powders
that can cure at temperatures below 100°C without compromising durability or quality.


Powder coating on wood is growing significantly. By developing powders with reduced heat
requirements and by developing a uniform density wood product, wood manufacturers and their
customers are now able to powder coat a wide range of wood products. Manufacturers of home-
office furniture, kitchen cabinets, children's furniture, and outdoor grill tables are discovering that
powder coating makes these "hard-use" products retain their new look much longer.


One of the biggest breakthrough in the wood market is the use of engineered wood materials
such as medium density fiberboard (MDF). MDF is very suitable for powder coating because of
its low porosity and homogeneous surface. Curing of powder on MDF can be accomplished by
infrared, or UV light in conjunction with infrared or convection ovens.


Recent advancements for industrial plastics applications
through Powder Coating

The growth of powder coatings has been dramatic during the last
decade with new applications continually being developed, including
plastics. Innovations in processes and materials have opened
previously unimaginable opportunities. Electrostatic magnetic Brush
(EMB) Coating is an innovative coating method for flat materials that
applies powder coating with roller technique, enabling relative high
speeds and a very accurate layer thickness between 5 and 100
micrometer. The base for this process is conventional copier
technology. Currently in use in some high-tech coating applications
and very promising for commercial powder coating on flat substrates
(Steel, Aluminum, MDF, paper, board) as well in sheet to sheet and/or
roll to roll processes. This process can potentially be integrated in any
existing coating line.


Future Trends
The success of conventional powder coatings is the result of their excellent properties and many
economic and environmental advantages over traditional solvent based paints. Many of the
disadvantages to the use of powders that had existed have been eliminated or minimized
through formulation and equipment development. These developments will help to assure that
powder coatings market will continue to grow at an impressive pace.


New resin systems allow powder to meet the end-user's specification for almost any product.
Many of these coatings can be cured at temperatures as low as 121°C. The advent of low
curing temperature systems, such as IR cured powders, has significantly opened up the market
to heat sensitive substrates such as wood, plastics, and assembled components with heat
sensitive details. The coating of metal substrates also benefits from this technology, with lower
energy and investment costs, shorter curing times, and higher lines speeds.


Powder coatings are also being developed for high temperature applications. Silicone-based
powder coatings are often used on products that must retain their appearance, adhesion, and
surface protection even after prolonged exposures to high heat (up to 538°C). Some of these
products include gas and charcoal grills, fireplace inserts, engine exhaust components, and light
fixtures.


Significant advancements are also being made in the weatherability of powders for use in
automotive and architectural applications. Polyester triglycidyl isocyanurate (TGIC) based
powders, for example, have been used on outdoor stadium seating and other exterior
applications that were previously susceptible to degradation from UV. The use of TGIC, which
has been labeled as toxic in certain regions, is now being replaced by other binders. Clear,
corrosion resistant, and durable powder coatings are used for a wide range of applications
including automotive parts. Auto manufacturers of high end vehicles are using powder clear
coats over automotive exterior basecoats.


Equipment development will also contribute to the future advancement of powder coatings.
Powder utilization rates of 95% or higher are common. This compares with a 30-80% utilization
rate for most other spray coating processes. Advances in infrared (IR) and ultraviolet (UV)
curing technologies are allowing increased production speed in powder coating facilities. IR
ovens can sometimes cure a part in as little as 30 secs, and UV curing can be accomplished in
a matter of seconds.


Thin films, 1-3 mils (25-75 µm), can be applied in a wide range of colours, glosses, and textures
and ultra thin film, 0.8-1.2 mils (20 - 30 µm) powders are currently being developed. These
powders offer better penetration into recesses, more film thickness control, and more effective
first pass transfer efficiency.


Colour and surface texture variety is almost limitless with new powder coating formulations.
Properly designed powder systems can now change colours in minutes. High production
powder systems apply over 20 different colours, with several colour changes per day.
Coil coating applications for powder are also being developed. Coil coating is the process of
coating one or both sides of flat metal sheets or strips on a continuous production line. Most
powder coating facilities are laid-out in a vertical configuration. However, newly designed
horizontal powder coating booths are enabling powder to compete more effectively in the
extrusion and pultrusion finishing markets.


Radically different methods of application are also likely to open future markets. In-mold powder
coating processes have been developed in which the powder coating material is sprayed onto a
heated mold cavity before the molding cycle begins. During the molding operation, the powder
coating chemically bonds to the molding compound and produces a product with a coating that
is chip and impact proof. Processes have been developed for applying thermoplastic powder in
the field, provided that the substrate is clean and can be preheated properly. Bridge support
columns and steel sidewalks have been coated successfully. Also, pipe joints that are originally
coated in the factory but must be welded in the field can be recoated with powder, thereby
providing a continuous, corrosion resistant coating.


Recent breakthroughs in powder coating technology have approached the mar performance
usually associated with porcelain enamels, as measured by commonly used industry tests. This
provides significant value to customers. The final properties of the powder coatings often are
superior to liquid coating systems. The reasons for the fast growth of this technology is that
powder coatings maximize production, cut costs, improve efficiencies, and offer maximum
compliance with increasingly stringent environmental regulations. The future for broad scope
powder coating is bright.


      Raman Gujral

      E-mail: gujralraman@yahoo.com

				
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