Powder Coatings: Process, Types, Benefits & Application

Introduction to Powder Coatings

The technology for coating products with dry powder rather than conventional liquids has been available since the 1950s. The powder used for the process is a mixture of finely ground particles of pigment and polymeric resin.

The powder is either:

Sprayed electrostatically onto a surface to be coated, or
The substrate is dipped into a fluidized bed of suspended powder

The powder adheres to a preheated substrate surface in the fluidized bed process, or they adhere electrostatically in the spray process. When heated further in a curing oven, the particles flow and fuse into a strong, adhering coating.

The result is a high quality coating with an attractive finish and excellent durability

Although the final properties of the powder coatings are often superior to liquid coating systems. The reason for the fast growth of this technology has been more related to the evidence that powder coatings:

Maximize production
Cut costs
Improve efficiencies, and
Offer maximum compliance with increasing stringent environmental regulations

Environmental advantages have led the way for the conversion of liquid coatings to powder coatings.

Key Benefits and Limitations of Powder Coatings

As compared to liquid coatings, powder coatings have been shown to offer several benefits as listed below.

They possess significant durability and resistance to abrasion, scratching, corrosion, and chemicals
They stay bright with less fading, and color selection is virtually unlimited with high and low gloss, metallic, and clear finishes available
Texture selections range from smooth surfaces to wrinkled or matte finishes, and rough textures are available for hiding surface imperfections
Thick coatings can be achieved quickly and efficiently
They contain no solvent, and thereby the process emits negligible, if any, polluting volatile organic compounds (VOCs) into the atmosphere
The processes used for powder coating do not require venting, filtering, or solvent recovery
Cost saving because there is less need for heating outside air to supply oven exhaust air, and
Most of the powder coating over-spray can be retrieved and re-used

Let’s check out the advantages and disadvantages of powder coatings in brief.

ADVANTAGES

DISADVANTAGES

No solvents so that VOCs are nil - minimal fire risk, environmentally friendly
Exhaust air from the coating booth can be returned to the coating room, thus less oven air is exhausted to the outside
Over-spray (up to 98%) can be retrieved and reused
No drying or flash time required so that parts can be racked closer together
Easily adapted to continuous, automatic processes
Coating does not run, drip, or sag, thereby lowering rejection rates
Minimum operator training and supervision
Thick coatings are easily possible
High throughput/output options
Simple clean-up and maintenance
Very high utilization rates of 95% or more
Operator training can be simplified
Generally reduced operator exposure risks
High film build in one coat
Very high performance in specific applications

Variable coating thickness may give optical defects, very thin coatings (less than 1.0 mil) are difficult because of pinholes
Frequent color changes could entail extensive downtime
Storage and handling of powder requires special climate controls
Color matching and color uniformity is more difficult than with liquid coatings
Uniformity of coating thickness is sometimes difficult to maintain
Cure temperatures required for some powders are too high for temperature sensitive parts
Conversion from liquid coating processes is expensive
Inside corners have low film thickness owing to the Faraday cage effect
Difficulty in coating sharp edges and inside corners of objects (less with tribo spray)
Contamination very easily produces surface defects
Difficulties in matting and obtaining some special effect finishes

Now when we have seen several benefits offered by powder coatings, it is equally important to understand its manufacturing and application process in detail.

How are Powder Coatings Manufactured?

The established manufacturing process for powder coatings comprises the following steps:

Dry blending all raw ingredients

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Passing through an extruder at elevated temperature to mill the pigments, melt the resins and achieve a homogeneous mix

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Cooling the extrudate rapidly

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Milling the flakes of an extrudate to final size in a multi-stage process

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Removing undersize and oversize particles

Limitations of Manufacturing Process

This process has a number of fairly evident limitations:

Liquid additives (including pigment dispersants) cannot be used unless first converted to a solid form.
Shearing time equals the dwell time in the extruder - typically around 20 seconds - with no opportunity to extend dispersion time or control it independently; the shear is inadequate for some types of pigments and too much for many effect pigments.
Care must be taken to ensure that extruder temperatures are low enough to avoid premature crosslinking. Various proposals have been put forward for processes which avoid these problems, but they have not so far had a major impact.

How are Powder Coatings Applied?

Powder coating is a dry finishing process, using finely ground particles of pigment and resin. These particles are generally electrostatically charged and sprayed onto electrically grounded parts.

The substrate is first pretreated similarly to conventional liquid-coated parts. The pretreatment process is normally conducted in series with the coating and curing operations. The charged powder particles adhere to the parts and are held there until melted and fused into a smooth coating in a curing oven.

There are essentially two common ways of applying powder coating:

Electrostatic spray
Fluidized bed

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 electophoretic deposition.

Let’s discuss the main technologies in detail,

Electrostatic Spray

Electrostatic spray method uses a powder-air mixture from a small fluidized bed in a powder feed hopper. 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. In some cases, the feed hoppers vibrate to help prevent clogging or clumping of powders prior to entry into the transport lines.

Electrostatic Spray Powder Coating Process

The electrostatic powder spray gun helps in:

Directing the flow of powder
Controlling the deposition rate
Controlling the pattern size, shape, and density of the spray
Charging 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.

There are various gun designs that mainly differ in the method of applying electrostatic charge to the powder. In some cases, the powder is electrostatically charged by friction. The advantage is that the powder is free to deposit in an even layer over the entire surface of the part, and deposition into recesses is improved.

The film thickness is dependent on the powder chemistry, preheat temperature, and dwell time.

Film thicknesses of 1.5 - 5.0 mils (37.5 - 125 µm) can generally be applied on cold products
If the products are preheated slightly, 20 - 25 mils (500 - 625 µm) coatings can easily be applied in a single coat

Fluidized Bed

The fluidized bed coating process is a simple dipping process that can be either conventional or electrostatic.

#1 Conventional 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 as shown in the figure.

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 i.e. 3 - 10 mils (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.

Effects of preheat temperature and dipping time on the film build in coating a steel bar with epoxy resin

#2 Electrostatic Fluidized Bed Process - It 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.

ADVANTAGES	DISADVANTAGES
Preheating of parts is generally not necessary Small products, such as electrical components, can be coated uniformly and quickly	The product size is limited Inside corners have low film thickness owing to the well-known faraday cage effect

How to Cure Powder Coated Products?

There are four basic methods normally used in the curing of powder-coated parts as discussed below.

Convection
- 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

Infrared
- 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

A combination of the two
- 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

Ultraviolet (UV) curing
- 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

Note: Thermoplastic powders require heat only to fuse the powder together into a continuous film while thermosetting powders often require additional heat to cure the film on the product

Below find a comparison of general curing conditions for heat curing of conventional thermosetting powders and UV curable powders using the various heating modes described above.

Heating / Curing	Temperature, °C	Total Curing Time, Minutes	Substrate
Conventional Thermosetting Powders
Convection	140-220	30-15	Metal
Infrared + Convection	140-220	25-10	Metal
Infrared	160-250	15-1	Metal
UV Curable Powders
Infrared / Convection for heating (1-2 mins) and UV for curing (secs)	90-120	3-1	Metal, wood, plastic, others

Formulating Powder Coatings for Temperature-sensitive Substrates

Types of Resins Used in Powder Coatings

The resin component in the formulation will largely determine the processing and end-use performance properties of powder coatings. There are two main classifications for powder coatings:

A thermoplastic powder coating melts and flows when heat is applied but continues to have the same chemical composition once it cools to a solid coating.
A thermosetting powder coating also melts when exposed to heat. However, after they flow to form a continuous film, they chemically crosslink on additional heating.

The final coating has a different chemical structure than the applied powder.

Thermosetting Powders	Thermoplastic Powders
Epoxy Epoxy-Polyester Hybrid Urethane Polyester Polyester TGIC Acrylic	Polyvinyl Chloride Polyolefins Nylon Polyester Polyvinylidene Fluoride

Thermoplastics Powders

Thermoplastic powders are typically high molecular weight materials that require high temperatures to melt and flow.

The primary advantage of thermoplastic coatings is that they form a smoother finish and require less energy
They are commonly applied by fluidized bed application, and the parts are both preheated and post-heated
Most of the thermoplastic powder coatings have marginal adhesion so that the substrate must be pretreated and sometimes primed prior to coating application

	Vinyl	Nylon	Polyester
Primer Required	Yes	Yes	Yes
Melting Point, °C	130-150	186	160-170
Typical Pre/Post Heat, °C	284 / 230	310 / 250	300 / 250
Adhesion	G-E	E	E
Surface Appearance	Smooth	Smooth	Slight Peel
Gloss, 60°	40-90	20-95	60-95
Pencil Hardness	HB-2H	B	B-H
Salt Spray Resistance	G	E	G
Weathering	G	G	E
Humidity	E	E	G
Acid Resistance	E	F	G
Alkali Resistance	E	E	G
Solvent Resistance	F	E	F
E = Excellent, G = Good, F = Fair

Physical and Coating Properties of Thermoplastic Powders

Thermosetting Powders

Unlike thermoplastic powder coatings, thermosetting powders are heat stable and will not revert back to the molten stage when reheated after curing. They are also:

Tougher
Have better adhesion to metal substrates, and
More resistant to solvents and chemicals

Thermosetting powders account for about 95% of all powder coatings.

	Epoxy	Epoxy Hybrids	Urethane Polyester	TGIC Polyester	Acrylic
Hardness (Pencil)	HB-7H	HB-2H	HB-4H	HB-4H	HB-4H
Impact Resistance (in-lbs)	60-160				40-100
Gloss (60° meter)	3-100+	10-100+	15-95	20-90	10-90
Color	Al colors, clear, textures
Salt Spray	1000 hrs	1000 hrs min	1000 hrs	1000 hrs min
Condensing Humidity	1000 hrs	1000 hrs min	1000 hrs	1000 hrs min
Cure Range*	3 min at 232°C to 25 min at 121°C	10 min at 204°C to 25 min at 149°C			10 min at 204°C to 25 min at 177°C
* Typical 2 mil (50 µm) film – time at metal temperature

All You Need to Know About Testing of Powder Coatings

Now let’s see some of them in detail.

Epoxy Powder Coatings

Epoxy coatings are used wherever a hard, electrically insulating coating is required to provide protection over a wide temperature range. Depending on the particular epoxy resin selected, these coatings can be used up to 150°C or higher.

Epoxy powder coatings have a dielectric strength of up to 1200 volts/mil., in thicknesses over 10 mils (250 µm). They are, therefore, ideal as functional electrical insulators as well as a protective coating

Chemical resistance to most solvents and mild acids and bases is good, and epoxy powder coatings provide excellent toughness and corrosion resistance. Adhesion to metal substrates is excellent, and generally epoxy powder coatings do not require a primer

Epoxies are often used for decorative applications. They can be formulated to provide a variety of colors, glosses, and textures. Typical applications include metal office furniture, shelving, interior car parts, and toys.

The major limitation of epoxy powder coatings is a tendency toward brittleness if the coating is over a few mils thick. They will also chalk when subjected to UV radiation. For this reason, they are rarely used for outdoor applications.

Epoxy-Polyester Hybrid Powder Coatings

Epoxy-polyester hybrids combine epoxy resin with polyester resin to form a powder with many of the same characteristics as the epoxies. This group of powder coatings could be considered part of the epoxy family except for the high percentage of polyester utilized (often in excess of half the resin).

Property wise these resins are very close to their epoxy counterparts:

Epoxy-polyester hybrid coatings are generally tough, flexible, and competitively priced when compared to pure epoxy coatings
Hybrids provide some improvement in weatherability, but they will begin to chalk almost as fast as an epoxy coating. However, after initial chalking, the deterioration is slower
Some hybrids are less resistant to chemicals and solvents. Hybrids are likely to be used in many of the same applications as epoxies

Urethane Polyester Powder Coatings

Urethane polyester powders are one of the two polyester powder coatings being used commercially (the other is polyester TGIC). The primary type that has been used for several years is a urethane cured polyester powder, which is comparable chemically to the exterior quality urethane.

Coatings of this type offer:

Outstanding thin film-appearance
Toughness with excellent weathering properties

It is common to block the crosslinker in urethane polyesters with e-caprolactam. To begin the crosslinking process, the material must reach a temperature above the blocking agent threshold. With e-caprolactam, unblocking occurs at approximately 182°C.

Urethane polyester powder coatings are true competitors to high-quality liquid paints
in respect to thin film appearance

They are used for exterior applications such as patio furniture, automotive wheels and trim, lawnmowers, and a wide range of other products requiring high-quality, decorative finishes.

Polyester TGIC Powder Coatings

Polyester triglycidyl isocyanurate (TGIC) powders are another type of polyester powder coatings being used commercially. In these coatings a very low molecular weight glycidyl or epoxy-functional curing agent is used to co-react with the polyester. In this way, the polyester constitutes a very high percentage of the resin and provides weather and corrosion resistance incomparable to the urethane cured polyesters. However, their resistance to chemical and solvents is lower.

TGIC powders have very good adhesion characteristics, corrosion resistance, and exterior durability
They can typically be cured at lower temperatures than urethanes and have shorter cure cycles
They also provide good edge coverage and tough, thick films

Typical applications of polyester TGIC powder coatings are where sharp edges and corners exist such as on automotive wheels, air conditioners, lawn furniture, and air conditioner cabinets.

Acrylic Powder Coatings

Acrylic powders provide good appearance, tough surface, exceptional weatherability, and excellent electrostatic application characteristics. Common acrylic based powder coats include:

Urethane acrylics
- Urethane acrylics require cure temperatures of 182°C
- They offer excellent thin-film appearance, good chemical resistance and hard films
- Flexibility and impact resistance is usually poor

Acrylic hybrids
- They combine the acrylic resin with an epoxy binder
- They are somewhat better than a polyester-epoxy hybrid but still not considered acceptable for exterior use
- Acrylic hybrids generally have much better mechanical properties than the other acrylic powder coats

Glycidyl methacrylate (GMA) acrylics
- GMA acrylics can be cured in less time or at a lower temperature than urethane acrylics, and they also can provide superior weathering characteristics
- They make excellent clear coats over brass or chrome
- Like the urethane acrylics, flexibility is somewhat limited

Polyvinyl Chloride (PVC) Powder Coatings

Polyvinyl chloride (PVC) coatings are used for their excellent properties, including

Softness, glossy surface & flexibility
Impact strength
Resilience, and
Salt spray and weathering resistance

PVC coatings have good chemical and water resistance. They can be formulated for:

FDA approved applications such as frozen food shelving and dishwasher baskets
General purpose electrical insulation
Chain link fence wire, outdoor furniture, and similar applications

Special primers are generally required for optimal adhesion. Two types of vinyl powder are used: dry blends and melt mixed. The dry blends are less expensive, but the melt mixed materials are superior in performance.

Polyolefin Powder Coatings

Polyethylene and polypropylene powder coatings provide soft and waxy films that have characteristics very similar to their plastic counterparts. Polyolefin powder coatings are used primarily for their low cost and ease of application.

However, they also have certain properties that are very attractive in many applications. In addition to toughness common to other thermoplastics, they have:

Low water absorption
Excellent chemical resistance
Good electrical insulation characteristics
High durability, and
Resistant to many chemicals and detergents; however, some solvents can break them down quickly

Polyolefins are often used to coat laboratory equipment because the surface is easily cleaned. They are also used for food handling equipment and for automotive applications such as battery hold-downs.

Nylon Powder Coatings

Nylon powders are practically all based nylon 11. Nylon 6/10 is sometimes used but at high cost. Nylon powders are used to produce powder coatings which of a wide range of benefits, including:

Toughness
Excellent abrasion, wear, and impact resistance
A low coefficient of friction
Good chemical and solvent resistance
Smooth surface finish

Generally, a primer must be used with nylon powder coatings to achieve the adhesion level needed for higher performance applications. Nylons can be formulated for food contact applications, outdoor applications such as light fixtures or seating and marine applications where they completely encapsulate fittings, bolts, and other hardware items.

Polyester Powder Coatings

Thermoplastic polyester powder materials have better than average adhesion properties without the need for a primer. They also exhibit good UV resistance and weatherability. Polyester powder coatings are somewhat more difficult to apply than nylon materials and are not as resistant to abrasion or solvent.

Polyester coatings are often applied to such items as outdoor metal furniture due to their good weatherability, corrosion resistance, and general durability.

Polyvinylidene Fluoride Powder Coatings

Polyvinylidiene fluoride (PVF) based coating resins have excellent weathering characteristics and excellent resistance to chemicals exception for certain hydrocarbon solvents. They are used to coat piping and valves used in the chemical process industries. Usually a chromate primer is recommended.

Powder Coating Resins – Start Selection and Request Sample »

Raw Materials Used in Powder Coatings

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-processable form.

The high performance of powder coatings is further increased by the use of additives. These additives are designed to improve properties such as:

Prevention of discoloration during powder coating resin synthesis and curing
Ease of spraying the powder onto the substrate
Enhanced durability of the coating during service life through light stabilization, and
Improved corrosion resistance

The main raw material components used in powder coatings include:

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

Tribo-charging additives such as antioxidants and light stabilizers are frequently used in powder coatings. Depending on their composition, tribo-static powders require additives, such as HALS to improve their charge ability in addition to their light stabilization properties.

Heat Stabilizers are required to be thermally stable and non-volatile during high-temperature powder extrusion and curing processes (electric and direct-fired gas ovens), and to show no discoloration. Various classes of antioxidants have different thermal stabilization mechanisms. High resistance against UV degradation is necessary for all exterior applications. This is achieved through a synergistic combination of UV absorbers and HALS.
- UV absorbers work by absorbing detrimental UV radiation and harmlessly releasing the energy as heat before it can cause polymer degradation.
- HALS neutralizes any free radicals, thereby preventing polymer degradation. HALS are not dependent on coating thickness for their efficacy.

The raw powder coating materials are generally mixed dry and then fed into an extruder for melt mixing. Once cooled, the extrudate is then chipped and ground into the final product. The chips are ground to a very specific particle size distribution depending on the application.

The particle size is important to the performance and appearance of the coating. The size of the powder particle can have an influence on the:

Behavior of the material in the delivery system
Charging systems, and
The final film characteristics

When reclaimed powder is used, the coater must maintain a consistent particle size distribution. There are several methods by which particle size distribution can be measured.

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