PRINCIPLES OF COATING WITH VINYL HEAT FUSIBLE POWDERS
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Robert Clark Thermoclad Co.
Powder Coating '94 Proceedings 371
ABSTRACT
For over 35 years Polyvinyl Chloride powder formulations have proven themselves as
functional, durable and cost efficient coatings, offering physical properties that are very
desirable for many applications. This presentation examines the properties and
characteristics of PVC using both fluidized bed, fluid head and electrostatic spray coating
techniques.
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1. PVC as a Dowder coatinv
The first powder coatings were applied by fluidized bed in 1952 by the German
Erwin Gemmer who later patented the process. By the early 1960’s commercial use of PVC
as a powder coating was well established, and now, for almost 40 years Vinyl, remains one
of the widest used thermoplastic powder coatings in the world.
PVC is recognized as a cost effective and corrosion resistant coating that applies at
a thicknesses of 8 to 40 mils using the fluidized bed or fluid head methods, and from 3-30
mils by the electrostatic systems.
Vinyl coating powders are generally manufactured in one of three processes: dry
blend, agglomerate mix or freeze grinding. A PVC compound is created from base resins,
plasticizers, stabilizers and pigments with a multitude of other additives to give the finished
product a variety of special properties.
The formulating flexibility offered by the different grades of PVC Base resins,
plasticizers and other additives give vinyl an almost unlimited range of physical properties
and characteristics to meet different end use requirements such as: chemical resistance,
compatibility at low and high temperatures, flexibility, impact resistance, weathering, salt
spray and abrasion. The hardness can range from soft and pliable to tough and rigid. Vinyl
can be compounded in an almost unlimited range of colors and glosses from matte to over
100% gloss.
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A range of physical properties for PVC Vinyl Powder Coatings:
Specific gravity of fused film
Bulk density of powder at rest
Fluidized
Hardness
Shore A
Shore D
Tensile Strength
Tear Strength
Elongation
Brittleness Temp
Maximum useful operating temp
Continuous
Intermittent
Melt Range
Volatility % Loss
60 min @ 230°F
4.5min @ 425°F
Weatherometer
Dielectric Strength
ASTM METHOD
D-792
D-1895
D-785
D-785
D-65 1
D- 1004
D-638
D-746
D-149
.52 - .62 g d c c m
38 lblcu ft
27 lblcu ft
60-100+
16-53+
1200-2800+ PSI
200-600 ppi
100-300%
[-20°C] -4'F
[87"C] 188°F
[176'C] 350°F
[ 190"-198"C]
375"-390"F
.3%
.17%
4,000 hrs. +
500-1,000 ~ 0 1 L d d
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2. Fluidized Bed Coating with PVC
The most common method to apply PVC is with Fluidized Bed. Generally, the Bed
is constructed as a two-compartment container, or tank with the open top, for dipping the
parts to be coated into the PVC powder fluidized within. The Fluidized Bed has an upper
chamber for storage and containment of the powder separated from a lower chamber by a
porous membranae. When compressed or blower air is introduced in the lower chamber
it creates a lifting effect on the powder, and "boiling" appearance of the powder.
Mechanical vibration and agitation of the bed is sometimes desirable to enhance fluidization
and reduce the possibility of air channeling and powder lumping.
One of the most important components of Fluidized Bed system is the supply of air
to the bed. This air must be clean, dry and free of moisture or oil that may contaminate
the powder, cause blocking and possible rupture of the porus membrane.
A standard Fluidized Bed Application procedure would be as follows: the parts are
first cleaned and the primer coat, (available in both solvent and water based form), is
applied; the part is then pre-heated to a metal temperature near 500°F and dipped in the
fluid bed; a post heat, usually at lower temperature, is then utilized to insure the proper
melt flow of the powder into a smooth and continuous film.
A vinyl homo-polymer formulation has no inherent self bonding properties, so it is
essential to use a primer to create a chemical bond to a substrate. The primers are usually
composed of a thermoplastic and thermoset resin composition, and can be applied by dip,
flow coat or spray methods.
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Some common applications of Fluidized Bed PVC are underground transformers,
fan guards, dishwasher racks, closet shelving, battery hardware, electrical conduit, irrigation
piping, sewer pipe, fence posts, and fabric, sign posts, glass lab ware, patio furniture,
shellfish traps, scuba gear, gabion mesh, marine hardware, boat bumpers, refrigerator
shelving, playground equipment, bicycle racks, dishdrain baskets, along with thousands of
other applications that require excellent resistance to corrosion and variety of chemicals.
3. Fluid Head Coating Process
The fluid head coating process is used mainly where it is not possible to dip the part
to be coated in a fluid bed, such as continuous wire or wire mesh, so in this process the
coating powder is pumped into a "HEAD" which brings the powder to the part that is to
be coated. This is done by using a pair of fluidized beds, one inside of the other to create
a "HEAD" of powder that the part passes through. The powder is pumped from the outer
bed or reservoir into the inner bed and the cycle continues. Some examples of products
coated in a fluid head process would be fence wire, rolled fence fabric, gabrion fabric,
continuous coating of pipe, small automotive hardware and many more parts where it is
faster or impractical to dip a part in a fluid bed and where an electrostatic spray process
is impractical.
4. Electrostatic Surav Process
This method is based on negatively or positively charging powder particles that are
then sprayed with pressurized air onto a grounded part to be coated. The charged particles
adhere to the substrate surface while the coating is fused in a post heat oven.
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Currently the most widely used powders for this process are thermoset powders,
such as polyester and epoxy, offering a thin decorative finish. Generally, these products
self-limitinp, characteristics and thicker coatings are unattainable or impractical. Vinyl
coatings, on the other hand, are slightly conductive and may easily build up coatings in
excess of 20 mils.
Electrostatic grade vinyl has for the most part the same physical properties as fluid
bed compounds, the primary difference is the coating thickness. Some powders are
sprayable at a thickness of 3-5 mils on wire mesh for example, although this may not be
possible on larger wire gauges and flat metal substrates. The average thickness can range
from 7-9 mils if desired as a minimum thickness properties such as salt spray resistance and
chemical resistance to acids and alkalies are maintained.
Spray grade PVC can be applied on both cold and preheated substrates and usually
require only a short post heat cycle. Most spray vinyl will fuse in essentially the same time
and temperature cycles as the thermosets, so most existing thermoset coating facilities may
readily apply Vinyl powder coatings.
Electrostatic vinyl can be applied using corona and tribo technologies and has had
positive results with electrostatic fluid bed. It is compatible with standard electrostatic
spray and reclaim systems and has been endorsed by the major equipment manufacturers.
PVC is a thermoplastic and does not require the long post heat cycle as do
thermosets. When vinyl coating is smooth and glossy, the process is complete.
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Some examples of application that electrostatic PVC has been used on are fabricated
wire displays, air filters, perforated metal screens, heavy gauge expanded metal for
architectural applications, rebar supports and a multitude of others.
5. SDecial Considerations When Electrostatic Coating With PVC Powders
Although the list of applications is extensive, there are a few considerations which
require attention when using electrostatic grade PVC.
Cross contamination is certainly a factor between thermoplastics and thermosets, but
standard cleaning considerations as those when changing colors, have been proven
sufficient.
PVC coatings require a primer to attain a chemical bond to metal or other substrates.
Environmentally friendly water based primers may be applied by various techniques prior
to powder application.
Self adhering PVC materials are available and may be considered as an alternative
to primer.
Due to larger particle size, vinyl powders will not “cloud” as easily in the booth as
do most thermosets (60-80 micron as apposed to 20-40 micron particle size). This requires
more fine tuning of the automatic spray equipment to achieve the desired coating. The
benefits of the larger particle size is ease of clean up and powder cloud confinement.
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Thick builds may cause powder blow off in an oven due to high air velocities or a
high post heat which may induce the thick mass of powder to expand quickly and almost
jump off the part in sections. These difficulties have been solved using graduated heat or
extended time and reduced temperature cycles and oven modifications. PVC also suffers
from the faraday effect during application as do all spray grade plastics, but due to
thickness and its ability to flow, it covers tight corners and weld points well.
An advantage offered with PVC is that the material may be formulated or
plasticizied to achieve certain desirable characteristics or properties. Incorporation of
compatible liquid plasticizers provide the coating flexibility and a degree of hardness as
required but it is also a source of volatile material. Although the percentage of volatiles is
commonly low (not exceeding 1% by weight for most compounds) it may be necessary to
treat this effluent by filtering, precipitation or incineration.
PVC FLUIDIZED BED, FLUID HEAD AND ELECTROSTATIC APPLICATIONS
Service Categorv ADDlication
Corrosion
Outdoor
Humidity
Marine
Electrical
Impact
Dishwasher Racks Concrete Reinforcement Sewer Pipe Interiors Fan Guards Battery Hardware Irrigation Housings Underground Pipe
Fencing Chicken Wire Pole Transformers Sign Posts Guard Rails Patio Furniture
Closet Shelving Hothouse Accessories Cooling Tower Hangers
Shellfish Traps Dock Ladders Scuba Tanks Boat Hardware Bumpers
Bus Bars Conduits Transformers Jumper Cables Cable Trays Mining Battery Boxes
Window Frames and Guards Stretchers Refrigerator Shelving Playground Equipment
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Abrasion
Flexibility
Decorative
Sound Insulation
Hog Flooring Bicycle Racks Tool Grips Work Gloves
Freezer Baskets Gifts
Display Racks Dishdrain Baskets Kitchen Accessories
Speaker Grills Speaker Baskets
General Parameters for Application of PVC Coatings
1. Substrate Pretreatment
Soil, rust, grease, oil and other substances must be fully cleaned from surfaces to
be coated to achieve maximum adhesion, end product performance and appearance. Normal
surface cleaning procedures may include solvent rinses, shot or grit blasting, and chemically
treating the substrate. Commercial chemical pretreatment systems are available, one of the
most common is a five-stage system, consisting of an alkali wash, water rinse, zinc or iron
phosphate bath, and a purifying water rinse. Non pretreatment system is common, the
number of wash stages will vary depending on the substrate to be cleaned. The following
methods, or a combination thereof, have been successfully employed; they are listed in
increasing order of effectiveness:
Vapor degreasing
Grit or shot blasting
Hot alkaline clean, water rinse
Sand blasting
Solvent clean, sandblast
Hot alkaline clean, rinse, acid pickle, rinse
Three stage iron or zinc phosphate system
Five stage iron or zinc phosphate system
Seven stage iron or zinc phosphate system
The rule of thumb is: the cleaner the part, the better the bond
After cleaning, avoid touching the surfaces and coat as soon as possible.
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2. Primers and their Application
When physical adhesion between the substrate and the vinyl powder is necessary,
the substrate should be coated with a vinyl primer. Primers may be applied by dipping,
spraying, flow coating or brushing. The preheat and fusion temperatures encountered in the
coating process may affect primer selection. Vinyl primers are available in both solvent and
water based versions.
Vinyl Primers can be purchased in a variety of solids ranges (from ready to use to
approx. 30% solids to reduce shipping costs) but are most commonly used at a 3 to 12%
solids content, depending on substrate mass, temperature and application system.
Dipping into a tankful of reduced primer is the most economical and practical
method of priming, however spray or a flow-coat may be more suitable for some
continuous conveyorized systems. After the part has been totally "wetted" by a primer. A
drain period and excess droplet removal is recommended to minimize subsequent blistering
in the preheat oven.
3. Jigging and Masking
In automatic or semi-automatic powder coating systems the parts to be coated must
be fastened or held to the conveying system. Jigs to hold the parts must hold them securely
but allow cleaning solutions and primers to drain away completely. The part should be
positioned in the jig to prevent over-coating or an excessive build-up in any one area.
When sections of a part are not to be coated, masking or special application systems
can be used. For masking, use heat-resistant, pressure sensitive tape or cold fixtures. Special
application procedures include designing the jig to cover part sections, partial immersion
in the fluidized bed, and leaving part sections unprimed to strip away after coating.
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4. Preheating the Substrate
Gas or electric ovens with recirculated convected heated air are the most common
method of bringing the substrate up to coating temperatures. Other processes include
infrared heaters and induction and resistance heating equipment. Preheating the substrate
crosslinks the primer to the part and prepares it for the top coat.
For good flow and controlled coating thickness, the temperature of the part must be
above the Vinyl powders' fusion temperature at the moment of dipping.
Preheating temperatures will be determined to fit each part's mass, configuration,
size, thickness and heat loss while moving the part from the ovens to the coating station.
Application temperatures may vary depending on the idiosyncrasies of the specific
compound, the substrate and oven equipment. Minimum part temperatures for dip or hot
spray electrostatic applications should exceed 450°F. (232°C) to realize practical production
cycles. For wire goods, a 5-10 minute preheat cycle at 550-650°F. (288-343°C) will suffice
to achieve proper temperature, where as lower and longer preheat temperatures would be
called for when coating heavier parts, or parts having substantial differential, such as
large springs, castings or complex parts combining tubular and wire goods. The coating
process generally slows down as the parts become more complex, where 10-30 minutes at
475-550'F. may be more suitable.
5. Dip Cycle or Spray Cycle
The longer that the preheated substrate is exposed to the powder, either by dipping
or spraying, the thicker the coating will build on the part. Care should be taken to give the
part a uniform powder application.
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In the fluidized bed system, a slight side to side or up and down motion of the part
during the dip will help the powder reach most of the hidden or hard to reach areas. In
most cases a dip time of 2 to 4 seconds will be sufficient, in some extreme cases where
very thick coating is desired a dip cycle of 10 to as much as 30 seconds may be required,
and in some extreme cases repetition of the preheat and dip cycles can be used to achieve
an exceptional coating thickness.
In electrostatic applications care should be taken for proper gun placement to give
a uniform coating; the line speed will have to be adjusted to give the part sufficient time
in proximity of the spray area.
6. Postheating
The times and temperatures needed to completely fuse vinyl powder vary with each
application. Parts with high mass and high heat temperatures can range from 350-700°F.
(177-371°C) for 5-15 minutes may be needed.
The same types of heating equipment can be used successfully for both preheating
and postheating.
7. Quenching
Quenching of the coated part is not always necessary, however, it is recommended
to prevent discoloration or loss of gloss for parts that have high heat retention. The most
commonly used methods are dipping or spraying ordinary tap water on to the fused part.
During this operation an additive may be needed to prevent water spots especially on dark
colored parts.
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Summary
Properly applied vinyl powder coatings produce uniform finishes that are free from
sags, tears or runs in a practical range of thicknesses from 7-40 mils with a single
application. Thinner or heavier coatings are possible by special coating techniques. During
the fusion or melting process, vinyl coatings develop a high degree of surface tension to
smooth out rough surfaces such as castings, welds, and forming imperfections. These
characteristics allow minimum surface preparation, aside from normal pretreatment, which
reduces preparation costs required for painted or other thin film powder coated surfaces.
The Future of PVC Powder Coating
Due to the excellent range of properties offered by the production process and the
many application methods available, the possible applications with PVC are almost
unlimited. A few innovative applications include concrete reinforcement, gabion coatings,
shopping carts, chicken wire, playground equipment, glass bottles (for holding acids and
light sensitive chemicals), speaker baskets (to reduce sound resonance), carbon fiber tubing
and mesh, and many automotive applications that require excellent corrosion resistance.
As a powder coating with over 35 years of industrial history, Polyvinyl Chloride
has proven its success commercially. As a thermoplastic, it offers unique properties
different from other protective and decorative coatings for applications requiring distinct
finishes and characteristics.
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Robert Clark is a Technical Sales representative with The Thermoclad Company,
a manufacturer of Heat Fusable Coatings, located in Erie, Pennsylvania. Prior to joining
Thermoclad, he managed several large manufacturing and finishing shops. Most recently,
he was Finishing Manager for the Lozier Corporation located in Joplin, MO. Bob has in
excess of 17 years experience in the application of Vinyl Compounds.