A. What Are Thermoset Powder Coatings?

A thermoset powder coating is one which results from the melting of resin-based powder particles under applied heat and their subsequent irreversible cross-linking into a hard continuous film.

Mechanism of Film Formation

Melting of powder particles  >>  Levelling (flow-out) & substrate wetting  >>  Polymerisation (cross-linking) at powder-substrate interphase  >>  Adhesion >>  Cured coating

Types of Powder Coatings

Our products are available in 5 broad groupings in terms of resin binder type and their applications:

Binder

Applications

A.  Epoxy

Interior

B.  Epoxy-polyester (Hybrid)

Interior

C.  Polyester

Exterior

D.  Polyurethane

Exterior
      

E.  Fluoropolymer

Exterior

For each of these binder types, there are various grades formulated to meet different requirements in terms of coating appearance, mechanical and chemical resistance properties, durability, functional requirements, and certain specific features, as well as powder curing requirements.

B. The Powder Coating Process

  1. Substrate surface treatment
  2. Application of the powder
  3. Curing of the powder

1. Surface Treatment

Powder coatings serve both decorative and protective functions for the substrates to which they are applied.  As protective coatings, they must have perfect adhesion to the substrate and be impermeable to atmospheric elements (humidity, oxygen, salts, aggressive chemicals etc), which can cause corrosion in the case of metal substrates, especially in aggressive or outdoor environments.

Therefore it is absolutely critical that prior to powder coating application, metal substrates must undergo the appropriate surface preparation.  A proper pre-treatment increases the surface energy and results in good substrate wetting as the powder melts and cures, giving satisfactory adhesion and consequently the required corrosion resistance.  The primary and most determining factor influencing its corrosion resistance is the quality of the substrate and its pre-treatment, while the powder coating plays a supporting role.  A good powder, no matter how well applied and cured, cannot make up for poor surface preparation that would predispose the substrate to the onset and propagation of corrosion in aggressive or outdoor environments.

For the majority of substrates in general, surface preparation consists of essentially 2 steps: cleaning and chemical conversion (pre-treatment).

Cleaning
Removes oils and greases, dirt and loose grime, rusts, mill scales etc.

Chemical conversion coating
This is a layer resulting from converting the metal surface chemically, so it is integrally bonded to the metal, providing an anchor for the powder coating, improving adhesion at the powder-metal interphase.  Can be either a spray or immersion system.

Substrate

Recommended Surface Treatment

1

Ferrous metals

(cold-rolled steel, cast iron etc.)

a.  Cleaning

b.  Iron phosphating (for indoor)

Zinc phosphating (for outdoor)     

2

Zinc surfaces

(hot-dipped or electrogalvanized

steel, zinc alloy casting etc.)

a.  Cleaning

b.  Zinc phosphating or chromating

3

Aluminium alloys

a.  Cleaning

b. Chromating or chrome-free conversion coating     

2. Powder Application

The 3 main application methods for applying powders are:

Corona (Electrostatic or Ionisation) Gun
The high voltage generated at the tip of the spray gun creates an electrostatic field between the gun and article being coated.  Powder articles flowing through become charged and are attracted to the earthed substrate and adhere to it.  This method is most widely used.

Tribomatic (Electrokinetic) Gun
Friction-charging.  Powder particles flowing through the spray gun are charged by friction rubbing against the inner wall of the transfer tube in the gun and are attracted to the earthed substrate as they exit the gun.

Fluidised Bed
Articles to be coated are preheated to a temperature above the powder’s melting point and dipped into the fluidised powder bed.  The powder melts and adheres to the article.  This method is used mainly for items requiring thicker coatings.

Depending on the application method used, the powders can be formulated and made for optimised spray characteristics ie. their intrinsic chargeability and particle size distribution. At the powder applicator’s end, the setting parameters of every component of the spray system (powder feed and hoses, gun, spray booth and earthing, compressed air etc.), whether manual or automated, must be optimised in order to produce good powder sprayability for consistent results.

3. Powder Curing

As is the case with baking a delicious cake made with a good recipe and ingredients, the final step in the powder coating process is crucial: powder curing (baking).

Requirements for optimal results

  1. Strict control of curing oven temperature:
    — minimal temperature fluctuations
    — good temperature homogeneity across different locations within the oven
  1. Matching of temperature and line speed settings (in the case of conveyorized ovens) to articles being coated:
    — settings need to be matched to substrate type, size and material thickness, based on the recommended curing schedule (metal temperature and time) for the particular powder being applied.

Consequences of incorrect temperature and/or dwell time in oven

  1. Under-curing or over-curing:
    — deviation in colour, gloss, finish from the standard
  1. Under-curing:
    — additionally, poor substrate wetting and levelling, and insufficient binder cross-linking, resulting in poor coating adhesion, leading to decreased mechanical properties, inferior chemical and corrosion resistance and poor durability.

Whether a manual, batch oven or a conveyorized oven is used for powder curing, the baking schedule must be completed in its entirety to ensure total cross-linking and the desired coating performance.

  • Powder spraying
  • Powder spraying 4
  • Coating big items
Note: Powder Coverage (m2 per kg)

A simple mathematical formula to calculate the coverage of powders in a typical electrostatic spray system:

Eg. for a powder with SG of 1.62 and applied at 70µm thickness, the theoretical coverage = 8.8 m2/kg

The formulation does not take into account spraying loss, which should be factored in if the actual practical coverage is required.  For the above example, for a spraying loss of say, 10% (90% usage efficiency), the actual coverage becomes 7.9 m2 /kg.

The loss factor is usually between 2-10% depending on the efficiency of the recovery system used.  The size, shape and contours of the articles to be coated also affect the amount of overspray.

C. Powder Coating Manufacturing and Quality Control

Coating powder manufacturing process entails the conversion of a mixture of different raw materials (resins, hardeners, pigments, additives, extenders) into the final product in the form of a homogenous, finely ground powder suitable for application on articles for decorative and protective purposes. Quality checks are performed at various stages in the manufacturing process to ensure the delivered powder product is easily applied at the powder applicator’s line and the final, cured coating performs according to technical specifications.

Powder manufacturing and QC flow: