Basic Overview of Powder Coatings

Contents

History

Powder properties

Chemistries

Manufacture

Application processes

Color

History

Early 1950s -first thermoplastic powders applied onto heated metal

Late 1950s -first thermosetting powders for pipeline protection (Shell)

1962 - dispersion via extrusion (Shell)

1964 - first epoxy-based decorative powder coatings

1964 - first dedicated powder spray equipment (Sames)

1970 - first polyester-based durable coatings. (Scado & UCB)

1970s oil crisis.

Growth rates from the 1970 - 1990s averaged 15% with peak growth rates often achieving 25%

2000 Growth in mature markets

Features of powder

Zero VOC

Comply with environmental legislation

H&S, reduced fire risk

Densely crosslinked coating, high Tg

Good barrier & adhesion properties

Easier equipment cleaning and removal of uncured overspray

Over-sprayed powder can be recycled

high usage rates (95%)

reduced air pollution & effluent

Usually cured by thermal energy

conventional powders unsuitable for heat-sensitive substrates like wood, plastics

Film thickness dictated by particle size

generally thicker than wet paint films (50-80m)

Coating Properties

100% Solids no VOCs

Thermosets 150-200C for 5-20 mins

Film Thickness 50-100m

Melt Viscosity 10000 mPas

Product Tg 45-55C

Film Tg 60-110C

Chemistry

99% thermosetting powders

Chemistry used in a product depends upon end use

requirements.

Main binder systems currently in use:

Polyester

UV resistance

Mechanical & chemical resistance only fair

Epoxy

Chemical resistance

Poor UV durability

Polyester/epoxy (hybrid)

Powder chemistries

Main cross-linking reactions used in powder coatings are as follows:

Epoxy - carboxylic acid

Too slow for ambient-cure coatings but very widely used in powders

PE/TGIC, epoxy-polyester hybrids and many acrylics all use this scheme

N.B. TGIC is a mutagen; TOXIC labelling now required.

Carboxylic acid - hydroxyl

e.g. PE/Primid

Hydroxyl - isocyanate

Polyurethane powders comprise OH-functional polyesters crosslinked with isocyanate adducts

Epoxy - amine

Epoxy - phenolic

Summary of Chemistries

Epoxy Epoxy

Polyester

Standard

Polyester

Superdurable PU Fluoro GMA

Acrylic

Acrylic

Hybrid

Exterior

Durability

Very

Poor

Poor Good Very Good Good Excellent Very

Good

Medium

Corrosion

Resistance

Very

Good

Very

Good

Good Good Good Good Good Good

Impact Very

Good

Very

Good

Good Poor Good Good Poor Poor

Flexibility Very

Good

Very

Good

Good Medium Good Good Good Good

Adhesion Very

Good

Very

Good

Good Good Very

Good

Medium Very

Good

Very

Good

Chemical

Resistance

Very

Good

Very

Good

Good Good Good Very

Good

Very

Good

Very

Good

Heat

Resistance

Very

Poor

Good Good Good Good Good Good Good

Abrasion Very

Good

Good Good Good Good Good Good Good

Manufacture

Pre-weighing of dry RMs

Extrusion (melt mixing & homogenising)

Micronising (milling)

Classification (particle size control)

Collect finished powder

Pre-mixing

Off-line tint

QC colour & gloss mechanical properties particle size

Powder Manufacture

Premix

Extrusion

Cooling

Kibbling

Milling

Sieving

Packing

Interpon

Application Fluidized Bed Methods

Hot dipping

Earliest means of application (1950s). Immerse pre-heated substrate in

fluidized bed of powder

good all-over coverage

thick film build, poor control

Electrostatic fluidized bed

Bed contains electrodes which ionize fluidizing air; this in turn charges

powder

No pre-heating; 30-100kV

good coverage

poor control of film build

Faraday cage effects

electrical safety problems

Fluidized bed application is now little used

Application Spray application

Most widely used means of application

Two types:

electrostatic (corona)

tribostatic (friction)

Spray application Corona spray Powder gun contains electrode at c.50-100kV

Ionized air molecules charge powder particles, which deposit on

earthed workpiece

Easy control of film build

Back-ionization - repulsion of free ions trapped in deposited powder layer

Faraday cage effects - powder particles cannot penetrate complex shapes

-V

f i e l d l i n e s

Spray application (contd.) Tribostatic (Tribo) Spray

Powder particles are charged by friction between each other and between

powder & gun

Magnitude of charge transfer depends upon:

materials used (triboelectric series)

intimacy of contact and residence time

particle size, shape and relative surface area

Tribo guns commonly have a PTFE lining (bottom of tribo series, all polymeric

materials charge +ve relative to PTFE)

No Faraday cage - only weak electric field

Back-ionization onset also delayed

High maintenance costs - gun wear

Lower powder throughput than corona

Difficulty charging fine powder

No Field Lines

+V

Formation of cured film

Powder particles are retained on the substrate by electrostatic attraction

until the coating is cured

Various types of curing oven can be used:

convection oven (most common)

medium-wave infra-red

induction

Film formation process:

Powder

particles Particles melt &

coalesce

Crosslinking &

hardening

DH DH

Typical Tests Gloss

Gloss measurements are made to

check that the gloss is within

allowable tolerances.

Typical gloss levels are:

Gloss over 80 %

Semi-gloss 66% - 79%

Satin 55% - 65%

Matt 20% - 30%

Typical Tests Film Thickness

The powder coating is formulated to

be used at a specified target film

thickness..

Typical Tests Adhesion by cross cut

A lattice pattern is cut through the coating

with a blade and tape iapplied over the

lattice and then removed rapidly.

Performance is measured by counting the

number of squares removed. Normal

standard is Gt0 (no removal on tape test)

if coating has been correctly applied and

cured fully

Typical Tests Pencil Hardness

Is a measure of the hardness of the

coating.

Performance is measured as the hardest

pencil to give no rupture to the film

Typical Tests Color Control

A spectrophotometer is used to assess

color.

Reflectance values are measured at

several wavelengths, computed and

calculations made in the computer.

Color differences are quoted in terms of:

Lightness DL

Red/Green Da

Yellow/Blue Db

Chroma (color strength) Dc

Hue Angle (shade) Dh

Lightness

Description of color

Lightness

Light colors

Dark colors

Chroma

Strong colors

Weak colors

Hue

Red, green, yellow, blue

Numerical Description of Color

CIEL*a*b* (Commission Internationale de lEclairage)

A Uniform Color Space

3 Dimensional

L* lightness/darkness

a* redness/greenness

b* yellowness/blueness

Color Difference from standard

Total difference form a standard

DE*>1 the color difference is perceptible.

Lightness difference:

DL* is positive, the trial is lighter than the standard.

DL* is negative, the trial is darker than the standard.

DE = {(DL*2) + (Da*2) + (Db*2)}1/2

Color Difference from standard

Difference on red-green axis:

Da* is positive or negative the trial is redder or greener than the standard.

Difference on yellow-blue axis:

Db* is positive or negative the trial is yellower or bluer than the standard.

Tolerancing in L*a*b*

Color is measured

Limits are set

Instrumentation

Spectrophotometer

Color fingerprint i.e. Reflectance spectrum