What is powder coating?

• “Paint in dry powder form”
• Mostly epoxy and polyester resins, or combinations of these
• The powder contains resin, curing agent, pigments and additives, all in solid form
• No solvents used in production or in application

Powder coating – production process

1. All the dry raw materials (in dry powder form) are pre-mixed
2. The mixture is fed into an extruder where some of the raw materials (binder) melt and all the ingredients are force-mixed further under high temperature and pressure
3. The mixture comes out of the extruder as a hot paste which is rolled flat, cooled and chopped into flakes
4. The flakes are fed into a mill and ground to a powder
5. The powder goes through a sieve to ensure the final powder coating matches the specified particle size before it is packed

Quality control, including colour matching, can only be carried out on the extruded product, but any adjustments / additions must be made before extruding.

Surface preparation

Objects to be powder coated must first receive surface preparation. Common methods are:

• Abrasive blasting
• Alkaline degreasing
• Iron phosphating or zinc phosphating
• Chromating

Application by electrostatic spray

Most often the objects are at ambient temperature when coated electrostatically, and baked in an oven at 160–220ºC for 5–20 minutes, where the powder changes:

• melting and flowing to a continuous film
• wetting the substrate
• gelling/ chemical curing

Objects can also be pre-heated prior to application by:

• convection oven
• electro induction coil
• infrared

The powder is blown softly from a high voltage powder gun (60 – 100kV) towards the object which is grounded (connected to earth). The powder will stick electrostatically to the object, which is moved to the oven. Typical material loss in an automatic plant: ≈ 5%

Application by fluidized bed (FBE)

Fluidized bed

A fluidized bed is a container that holds the powder material with an air chamber at the bottom referred to as an inlet plenum.

The container and the plenum are separated by a membrane that is porous enough for air to pass through but not porous enough for solids to pass through. Compressed air is introduced into the plenum and up through the fluidizing membrane.

As the compressed air passes up through the container, the powder particles are suspended in the airstream. In this suspended state, referred to as fluidization, the powder/air mixture behaves somewhat like a liquid.

Application

• The object is pre-heated to 220-240°C
• Dipped in the fluidized powder for a certain time
• The film is formed while dipping
• Curing takes place by residual heat in the object

Advantages

• Simple process
• High DFT (400-500 µm) possible in one operation
• Complete coverage of complex geometries

Suitable items must have high mass relative to size

• In order to retain temperature long enough to build coating thickness
• To facilitate complete curing after withdrawal from the fluidized bed

A typical powder coating application plant

Typically used as part of production line (conveyor belt):

• Cladding, sheets and aluminium profiles
• Domestic appliances
• Steel pipes
• Reinforcing bars for concrete (fusion bonded epoxy, FBE)

• Tank lining is one of the most critical coating application jobs in the coating industry and needs a lot of attention during application in order to minimize future claims
• A general definition for tank lining is a material (coating, glass fibre lining, rubber lining, etc.) which is applied to interior surfaces of tanks that require protection from exposure to strong chemicals / products contained in the tank
• Purpose of tank lining:
  • Protect the tank content from contamination
  • Protect the tank against corrosion and undesirable effects from the content
  • Provide easy cleaning of the tank

Common generic types of tank coatings

• Epoxy (pure)
• Phenolic epoxy
• Inorganic zinc silicate
• Vinyl-ester

Both surface preparation (min. Sa 2.5) and application must be of highest quality. Inspection must be thorough and detailed, with particular attention to ambient conditions, DFT, pin-hole detection, drying & curing conditions, etc.

The liquid tank-content will always penetrate the tank coating to a larger or lesser degree, and some swelling of the film may take place. This is normal and it will not cause any problem for a suitable and good tank coating. Once the tank is empty and ventilated, the liquid will leave the coating film and it will be reconstituted. If the liquid content is not volatile, cleaning of the tank will be necessary before the tank is filled again with a different liquid content. Insufficient cleaning will not only result in contamination of the next tank-content, it may cause coating failure if the new tank-content is not compatible with (or cause a reaction with) remains of the previous tank-content which still is embedded in the coating film.

Two different tank scenarios

• Storage tanks
  • The content type is predominantly the same all the time
  • The coating must be resistant to long-term exposure to the particular content

• Cargo (transport) tanks
  • The content may often differ from one transport to another
  • Cleaning between different cargo types may be crucial
  • Sequencing (order) of different cargoes is important, since cargo-remains in the film must be compatible with the following cargo

• Types of glass:
  • Flakes
  • Woven mats
  • Non-woven mats
  • Chop-strand from chopper gun
• Can be combined with both epoxy, polyester and vinyl-ester resins
• Used inside tanks, usually at the bottom / lower sides
• Requires Sa 2½ steel preparation

Glass fibre lining – application

• Resin is usually applied directly on the prepared steel surface, the glass fibre mat is pressed into the wet film and more resin applied on top
• Chop-strand application may also be used
• System may be built up of several coats
• High labour cost
• High material cost

Instead of using mats of glass fibres, a Chopper Gun can be used to automatically chop and deposit fiberglass filaments. The Chopper Gun makes it easy to quickly cover large surfaces with fiberglass. Strings of fiberglass are fed through the back of the gun in a long continuous piece. An adjustable blade assembly controls the size of the fiberglass strands it cuts, and the trigger controls the speed at which these strands are ejected from the gun and onto the surface. This chopper gun is used together with an airless spray gun, so both fibre glass and resin are applied simultaneously.

• Used for tanks, pipe sections, vales and chutes etc.
• Some grades used for abrasion resistance
• Natural rubber lining is typically used in storage tanks for concentrated hydrochloric acid (HCl)
• Film thickness up to several centimetres
• System consists of primer, adhesive and rubber sheets
• Made from different types of rubber such as Hypalon, Neoprene, Natural rubber

Rubber lining - application

• All steel defects should be remove thoroughly, and surface must be smooth
• Abrasive blasting to Sa 2½
• Apply a system:
  • Primer
  • Adhesive
  • Rubber sheet
• Cure by vulcanization

Vulcanization: a physiochemical reaction to form cross linking of the rubber with sulphur and heat.

• Limited size and shape of items to be coated
• Mostly used on pipes
• No solvents
• Long runs can require special application equipment
• Applied by wrapping over a primer
• Thickness from 1 mm upwards
• No curing

A number of different types of tapes is available, each designed for their particle use:

• Polyethylene tapes
• Petrolatum (greasy) tapes
• Elastomerised Bitumen tapes
• PE-Butyl Rubber tapes
• High temperature tapes

• Coatings that do not dry or cure, but stay soft all the time
• Protect against corrosion by forming a barrier
  • Wax (warm applied)
  • Grease (Lanolin / Wool fat / Sheep grease), very messy!
  • Bitumen (hot applied)

• Usually very good penetrating properties
• Applied by spraying or brush
• Very difficult to change to a drying type of coating (very difficult to clean/remove, causing adhesion challenges for drying type coatings)
• Used in cofferdams, void spaces, ballast tanks, under-carriage of cars & trucks

Flow coats were used some time ago: The oil-like coating was poured into the bottom of a water ballast tank and water was carefully filled into the tank. The coating would float on top of the water and a layer of the flow coat would be deposited on the tank walls as the water level in the tank raised. It provided a limited protection against corrosion and had to be repeated after a few ballast sequences. It is no longer in use due to poor performance and high sea-pollution consequences.

A number of different special coatings contain glass flakes as reinforcement. The two main reasons would be to increase the coating’s abrasion resistance (e.g. for use on the hull of ice-breaking vessels) or its chemical resistance (e.g. for tank coating). Typical binders used for glass flakes containing coatings are epoxies, polyesters, vinyl esters, etc.

Glass flake vinyl ester paint

Advantages

• Very fast curing
• Very good adhesion
• Very good abrasion resistance
• Very good chemical resistance
• Very good solvent resistance
• Can be applied by normal airless spray
• Glassflakes improve abrasion resistance

Limitations

• Short pot life (approx. 45 min.)
• Overcoating interval: Min. 2 hours, max. 24 hours
• Limited shelf-life: 6 months at 23˚C
• Dependent on temperature during application
• Only to be applied on blast-cleaned steel (Sa 2½ )

Note: the peroxide catalyst has high risk of self-ignition

Most organic binders start breaking down at temperatures above 250°C. Special heat resistant paints will have inorganic binders which can tolerate temperatures above 250^oC over longer periods.

The main purpose of heat resistant paints is to protect heated surfaces against corrosion (as well as giving some visual benefits).

Most heat resistant paints shall be applied at ambient temperature, but some allow maintenance painting without shut-down of hot processes.

Common types of heat resistance paints are:

• Aluminium silicone
• Zinc silicone
• Inorganic zinc

Temperature limit

Heat resistance is dependent on choice of binder, and the metal contained. Typical acceptable limits:

• Aluminium silicone:  600^oC
• Zink ethyl silicate:  400^oC
• Silicone acrylic:  250^oC

Heat resistant paints - application

• To be applied only on Sa 2½
• To be applied in multiple thin coats, typical DFT 20 µm, heavier coats may give blistering
• Zinc ethyl silicate must be fully cured before over coating