principles of coating with vinyl heat fusible powdersinfohouse.p2ric.org/ref/28/27189.pdf ·...

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.

312 Powder Coating '94 Proceedings

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.

Powder Coating ‘94 Proceedings 373

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


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.


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.


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.


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.

378 Powder Coating ‘94 Proceedings

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

380 Powder Coating ‘94 Proceedings

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.


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.


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.


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.


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.


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.

General Session

NEW TECHNOLOGIES & CASE HISTORIES

Powder Coating ‘94 Proceedings

principles of coating with vinyl heat fusible powdersinfohouse.p2ric.org/ref/28/27189.pdf ·...

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