ReBar Coating

Fusion bonded epoxy coating, also known as fusion-bond epoxy powder coating and commonly referred to as FBE coating, is an epoxy based powder coating that is widely used to protect concrete reinforcing bars (re bar). FBE coatings are thermoset polymer coatings. The name fusion-bond epoxy is due to resin cross-linking and the application method, which is different from a conventional paint. The resin and hardener components in the dry powder FBE stock remain un-reacted at normal storage conditions. At typical coating application temperatures, usually in the range of 180 to 250 °C (356 to 482 °F), the contents of the powder melt and transform to a liquid form. The liquid FBE film wets and flows onto the steel surface on which it is applied, and soon becomes a solid coating by chemical cross-linking, assisted by heat. This process is known as “fusion bonding”. The chemical cross-linking reaction taking place in this case is irreversible. Once the curing takes place, the coating cannot be returned to its original form by any means. Application of further heating will not “melt” the coating and thus it is known as a thermoset coating.

Fusion bonded epoxy coating application process
Regardless of the shape and type of steel surface to be coated, the FBE powder coating application has five essential stages:

• State 1: Surface Preparation – ReBar Shot Blasting Process (Wheel Blast)
• Stage 2: Pre Heating to recommended FBE powder application temperature. (Induction heating)
• Stage 3: Coating - Fusion Bonded Epoxy Coating.
• Stage 4: Curing stage – Water Quenching.
• Stage 5: Inspection
  

The advantage of rebar is that their round shape allows continuous linear application over the exterior surface, while the parts are moved in a conveyor through the powder application booth, ensuring high throughput.

Stage 1: Surface preparation — Airless (Wheel Blast) Shot Blasting Process

Shot Blast cleaning is the most commonly used method for preparation of steel surfaces. This effectively removes rust, scale, slats, and makes surface rough (rough surface is called profile). Profile increases the effective surface area of the steel. The cleanliness achieved is assessed to by Swedish standard from a set of photographic slides in Swedish standard (SIS) showing examples of the common terminology of white-metal, near white-metal, etc. Typically, SA 2½ is used for pipelines (equivalent to NACE N°2).
It is important to remove grease or oil contamination prior to blast cleaning. Solvent cleaning, burn-off, etc., are commonly used for this purpose. In the blast cleaning process, a blasting wheel uses centrifugal force to propel the abrasive against rebar in controlled direction, speed and quantity. The equipment is equipped with 4 blast wheels. 2 blast wheels are mounted on the top of the cabinet and positioned above the pass line of the work and the other 2 Blast wheels are mounted in the bottom of the cabinet positioned below the pass line of the rebar. Blast Wheel position and angel are strategically designed in such a way that the abrasive blast pattern covers the entire surface multiple re-bar.

Stage 2: Pre Heating – Induction Heating
Heating can be achieved by using several methods, but the most commonly used ones are ‘’induction heating’’. Induction heating is the process of heating an electrically conducting object (usually a metal) by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal. An induction heater (for any process) consists of an electromagnet, through which a high-frequency alternating current (AC) is passed. Heat may also be generated by magnetic hysteresis losses in materials that have significant relative permeability. The frequency of AC used depends on the object size, material type, coupling (between the work coil and the object to be heated) and the penetration depth.

The steel part is passed through a high frequency alternating current magnetic field, which heats the metal part to the required FBE coating application temperature. Typical application temperature for a standalone FBE is 225° to 245°C. When used as a primer in a multi-layer polyolifine system, application temperature may be dropped based on FBE manufacturer's recommendations, in order to meet the "inter-coat adhesion" parameters.

Stage 3: Fusion bonded epoxy coating
The FBE powder is placed on a “fluidization bed”. In a fluidization bed, the powder particles are suspended in a stream of air, in which the powder will “behave” like a fluid. Once the air supply is turned off, the powder will remain in its original form. The fluidized powder is sprayed onto the hot substrate using suitable spray guns. An electrostatic spray gun incorporates an ionizer electrode on it, which gives the powder particles a positive electric charge.
The steel to be coated is passed through the powder stream on conveyor. The charged powder particles uniformly wrap around the substrate, and melts into a liquid form. The molten powder ‘flows’ into the profile and bonds with the steel. The molten powder will become a solid coating, when the ‘gel time’ is over, which usually occurs within few seconds after coating application. The resin part of coating will undergo cross-linking, which is known as “curing” under the hot condition. Complete curing is achieved depending on the FBE coating system, full cure can be achieved in less than one minute

Stage 4: Water Quenching
The coated bars after curing are passed through a water tunnel.  Water is sprayed on to the bars to cool them.

Stage 5: Holiday Testing
Once the rebar has passed through the cooling tank, it is tested with a holiday detection device that electrically examines the rebar for minute cracks or pinholes in the coating. If a holiday is detected, the area is marked for easy identification during the touchup procedure. The rebars are conveyed from the holiday detection device to a rack area where the holidays are touched up.
After the rebar are repaired, they are bundled and readied for shipment.