Introduction To Fusion Bonded Epoxy Coating And Its Process
Fusion Bonded Epoxy is very fast curing, thermosetting Protective Powder Coating. It is based on specially selected Epoxy resins and hardeners. The epoxy formulated in order to meet the specifications related to protection of steel bars as an anti-corrosion coating.
FBE is widely used in Rebar coating. It is the preferred over other protective systems, due to their several advantages. It acts as a barrier to corrosive chemicals and moisture. It is applied on preheated steel as a dry powder, which melts and cures to a uniform coating thickness.
The corrosion control property of the coating is dependent on its ability to be an excellent barrier against water, oxygen, chloride, and other aggressive elements thru concrete to the metal surface. However, for a coating to be an effective long-term corrosion protection system, it is essential that it stays bonded to the substrate during the entire life of the structure.
ADVANTAGES OF FUSION BONDED EPOXY
The most important requirements of the coating system for steel reinforcing bars are the following:
- Strong adhesion to the surface.
- Long-term chemical and mechanical resistance of coating materials at all temperatures during all seasons under working environment.
- High mechanical impact strength.
- Penetration resistance to all aggressive chemicals.
The Rebar coating system is equipped with a wide range of performance properties:
- Low permeability to water vapor and gas pressures.
- Mechanical protection against handling and transportation damage.
- Superior resistance to disbonding.
- Outstanding dielectric properties.
- Aging resistance against heat and resistance against oxidation damage.
- Strong resistance to ambient conditions such as corrosive soils, salt water, Micro-organisms and penetration of plant roots.
Surface preparation is essential to the ability of the coating to bond to the Rebar surface. This bonding is important to eliminate the environmental fluid migration between the substrate and the rebar coating.
The surface is cleaned using steel grit to obtain surface profiles of 40-110 microns. During blast- cleaning the bar surface temperature shall be more than 3 °C above the dew point. The bar surface temperature shall always be more than 5 °C. The relative humidity shall not be greater than 85%. Abrasives shall be stored and used dry.
Reinforcing bars are blast-cleaned to a near white metal finish using abrasive grit in Shot Blaster. The shots clean the surface of the bar. The grits provide an anchor (40 – 100µm) to the bar surface, which cleans the steel of contaminants, mill scale, and rust. It also roughens the surface to give it a textured anchor profile. During this process, salt and other mineral contamination are removed.
The bars moved at a scheduled speed. Uniform heating is of paramount importance for good coating properties. Induction heating is the only method for this coating process. It is essential to serve a clean oxide-free surface to the heater in order to obtain a constant application temperature.
FUSION BONDED EPOXY POWDER APPLICATION
The heated steel is passed through a powder-spray booth where the dry epoxy powder is emitted from a number of spray nozzles. As the powder leaves the spray gun, an electrical charge is imparted to the particles. These electrically charged particles are attracted to the grounded-steel surface providing even coating coverage. When the dry powder hits the hot steel, it melts and flows into the anchor profile (i.e., the microscopic peaks and the valleys on the surface) and conforms to the ribs and deformations of the bar. The heat also initiates a chemical reaction that causes the powder molecules to form complex cross-linked polymers which give the material its beneficial properties The melted epoxy resin reacts with the curing agent present in the FBE and bonds to the substrate, providing a highly cross-linked polymer with a sophisticated network of covalent and coordinate bonds. These high – energy bonds provide excellent adhesion between the coating and substrate.
The amongst three adhesion forces between FBE and substrate, the two of them i.e. chemical and polar-polar adhesion are directly related to the number of bondable sites available on the substrate. Therefore to provide the maximum bondable site to FBE the high peak heights are obtained by abrasive cleaning. It also depends upon the viscosity of the FBE system used and the application temperature.
FBE has to achieve at least 97% cross-linking (cure) in order to have the optimum properties. The curing process is a heat-related phenomenon. It depends on the initial steel temperature and the duration that the bar retains the heat. Even if the initial steel temperature is low, you can reduce the bar travel speed in the coating line to allow enough heat energy for the curing process to be completed.
The coated bars after curing are passed through a cooling tunnel, In this tunnel, water is sprayed on to the bars to cool them. The bars are then kicked off on to the final inspection rack for testing and inspection. The testing on coated bars is carried as per ASTM775 and IS 13620 or such specification requirements. The acceptance tests usually conducted are thickness measurements, flexibility test, and holiday test. Once they are inspected they are bundled/strapped for dispatch to the job site for fabrication.
BENEFITS OF EPOXY-COATED STEEL REINFORCING BARS
- Excellent Corrosion Protection.
- More than 35 Years of Experience.
- Extended Service Life.
- Cost Effective Life-Cycle.
- Low Initial Investment