7/28/2019 kap1-2

http://slidepdf.com/reader/full/kap1-2 1/5

I

Powder Coater’s Manual 1/98I/6

In addition to the toughness and chemical resistance common to otherthermoplastics, polyethylene is an excellent electrical insulator. It alsois used to coat laboratory equipment because the surface is very clean-able.

Nylon

Nylon resins (polyamides) produce powders that are tough, smooth,chemical and solvent resistant, and very abrasion resistant. Most of the time a nylon powder will require a primer to achieve the adhesionlevel needed for higher performance applications. Nylons can be for-

mulated for food contact applications, they make an excellent coatingfor shelving and they are also widely used on bearing surfaces to pre-vent breakdown from mechanical abrasion. Some nylons are also usedfor outdoor applications such as light fixtures or seating.

Polyester

Polyester-based thermoplastic powder materials have better than av-erage adhesion properties and good UV resistance characteristics.However, they are somewhat more difficult to apply than nylon mate-rials and not as resistant to abrasion or solvent. Polyesters are used onsome outdoor applications such as patio furniture due to good out-

door durability characteristics.

PVDF (Poly-Vinylidene Fluoride)

PVDF based coating materials have excellent weathering characteris-tics. They also have excellent resistance to chemicals with the excep-tion of hydrocarbon solvents. They are used to coat piping and valvesused in chemical process industries. Usually, a chromate primer isrecommended.

2 Thermoset Powders

 Thermosetting powders are much more widely used than thermoplas-tics. They are primarily composed of relatively high molecular weightsolid resins and a crosslinker. Thermoset powders are used for a widevariety of decorative and protective applications. They are applied bythe electrostatic spray process, heated to the necessary temperatureand cured.

 Thermoset Powders

7/28/2019 kap1-2

http://slidepdf.com/reader/full/kap1-2 2/5

I

Powder Coater’s Manual 1/98 I/7

 The primary resins used in the formulation of thermosetting powdersare:

• Epoxy

• Polyester

• Acrylic

 These primary resins are used with different crosslinkers to produce avariety of powder materials. Many crosslinkers, or cure agents, areused in powder coatings, including amines, anhydrides, melamines,and blocked or non-blocked isocyanates. Some materials also use more

than one resin in hybrid formulas.When a thermoset powder is applied and subjected to heat it will melt,flow and chemically crosslink to form a finished film. The chemicalreaction in the cure cycle creates a polymer network that providesexcellent resistance to coating breakdown. A thermoset powder thathas cured and crosslinked will not melt and flow again if subjected toheat a second time.

Epoxy

Epoxy powders were the first commercially available thermoset mate-rials and they are the most commonly used of the thermosetting pow-ders today. They are available in a wide range of formulations forthick film functional applications and thin film decorative applications.

 They provide excellent toughness, chemical resistance, corrosion re-sistance and flexibility. The primary drawback with epoxies is thatthey will chalk when subjected to UV radiation. For this reason theyare rarely used for outdoor applications.

Functional Epoxy Powder Applications

Functional epoxies are commonly used for electrical insulation andcorrosion protection. Some typical applications for functional epoxies

are electric motors, alternators and electrical control junction boxes,taking advantage of its insulating properties. Because functional epoxiesalso have very good chemical resistance, they are used in a variety of applications where corrosion resistance is required, such as piping,under the hood automobile components and concrete ”rebar.“

 Thermoset Powders

7/28/2019 kap1-2

http://slidepdf.com/reader/full/kap1-2 3/5

I

Powder Coater’s Manual 1/98I/8

Decorative Epoxy Powder Applications

Epoxies are often used for decorative applications. They can be for-mulated to provide a variety of glosses, colors and textures and ap-plied in relatively thin films of 0.5 to 3 mil, still providing the tough-ness and durability of the functional coatings. Typical applications in-clude metal office furniture, shelving, interior car parts, and toys.

Epoxy Polyester Hybrids

Epoxy-Polyester ”Hybrids“, like the name implies, combine epoxyresins with polyester resins to form a powder with most of the same

properties as epoxies. Although some hybrids are less resistant tochemicals and solvents, they are tough, flexible and competitivelypriced.

Hybrids are likely to be used in many of the same applications asepoxies. In addition to the slight improvement in weatherability, thepolyester resin sometimes provides some improvement in chargingcharacteristics for electrostatic application.

Polyester Powder

Polyester resins are used to formulate urethane polyesters and polyes-

ter triglycidyl isocyanurate (TGIC) materials.

Urethane Polyesters

Urethane cured polyester powders have excellent resistance to out-door environments, toughness and very good appearance character-istics at 1 to 2 mil film thickness. A smooth, thin film that resistsweathering and physical abuse makes the urethane polyesters a popu-lar finish for high quality products.

It is common to block the crosslinker in urethane polyesters with e-caprolactam. To begin the crosslinking process, the material must reacha temperature above the blocking agent threshold. With e-caprolactam,unblocking occurs at approximately 360 °F (182 °C). Therefore, tem-peratures must be higher than 360 °F to start the melt phase of thecure cycle.

Powders containing e-cap are usually sensitive to films thicker than 3mils. Thicker films with these urethanes may lose some of their me-chanical properties and they may exhibit outgassing effects due to e-caprolactam evolution.

 Thermoset Powders

7/28/2019 kap1-2

http://slidepdf.com/reader/full/kap1-2 4/5

I

Powder Coater’s Manual 1/98 I/9

 They are used for exterior applications such as patio furniture, auto-motive wheels and trim, lawnmowers and a wide range of other prod-ucts requiring high quality, decorative finishes comparable to wet coat-ings.

Polyester TGIC

Polyester TGIC coatings use the epoxy functional crosslinker triglycidylisocyanurate (TGIC). TGIC’s have very good adhesion characteristics,corrosion resistance and exterior durability. They can typically be curedat lower temperatures than urethanes and/or have shorter cure cy-

cles. In the cure cycle, they have good overbake characteristics andthey are less sensitive to outgassing. They also provide good edgecoverage and tough, thick films (3–5 mil/ 75–100 µ) due to the inher-ent higher melt viscosity of the non-blocked TGIC crosslinker. Theyhave equal corrosion resistance to urethane polyesters but they aresomewhat less resistant to chemicals.

Some typical applications are automotive wheels, air conditioners,lawn furniture, and air conditioner cabinets.

Acrylic Powders

Like the polyesters, acrylics give excellent exterior durability. Com-mon acrylic-based materials include urethane acrylics (hydroxyl func-tional resins), acrylic hybrids (acid functional resins) and glycidyl meth-acrylate acrylics (GMA) (epoxy functional resins).

Urethane acrylics require cure temperatures of 360 °F (182 °C). Likeurethane polyesters, they may exhibit problems with outgassing atthicker films (3 mils/75 µ). They offer excellent thin film appearance,good chemical resistance and hard films. Flexibility and impact resist-ance is usually poor.

GMA acrylics can be cured in less time or lower temperatures than the

urethane acrylics and they also can provide superior weathering char-acteristics. They make excellent clear coats over brass or chrome dueto their exceptional clarity. Like the urethane acrylics, flexibility is some-what limited. They can be applied in films greater than 3 mils (75 µ)without the risk of outgassing that is characteristic with the urethanes.GMA acrylics are typically not compatible with other resins and arerun in systems that are isolated from other powders.

 Thermoset Powders

7/28/2019 kap1-2

http://slidepdf.com/reader/full/kap1-2 5/5

I

Powder Coater’s Manual 1/98I/10

Acrylic hybrids combine the acrylic resin with an epoxy binder. Theyare somewhat better than a polyester/ epoxy hybrid but still not con-sidered acceptable for outdoor use. The mechanical properties thatare characteristic in epoxies are a benefit of these materials and theyhave much better flexibility than other acrylics.

Because of their good appearance, tough surface, exceptional weather-ability, and excellent electrostatic application characteristics, acrylicsare frequently used for applications on products that have very highquality standards. Appliances, automobiles and other products thatrequire durability and long life in a harsh environment are good candi-

dates for acrylic powder coatings. Typical applications include auto-motive wheels, plumbing fixtures and vending machines.

Research has been conducted to determine the suitability of acrylicpowder coating as a clear coat on automotive bodies. While the UnitedStates automobile manufacturers continue to evaluate this applica-tion, one European manufacturer is using it in production.

3 Coating Comparison

 The decision on what powder to use depends on the specific appear-

ance and performance requirements of the end product. The end usermust determine what specific properties they want in the coating andcommunicate that to the formulator.

When selecting a powder, several variables must be considered.

• Cost of the powder (applied)

• Performance characteristics

• Application characteristics

• Appearance

 The proper coating is a balance of these variables. Coating manufac-turers can help the applicator in the selection process. Given a par-

ticular specification, they can often formulate a powder to meet aspecific application at a reasonable cost, particularly if the volume ishigh.

 The comparison charts below show some of the performance proper-ties and typical uses of the various resins.

Coating Comparison