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Fusion bonded epoxy coating
Additional recommended knowledge
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 various sizes of steel pipes used in pipeline construction, concrete reinforcing rebars and on a wide variety of piping connections, valves etc. from deterioration due to corrosion. FBE coatings are thermoset polymer coatings. They come under the category of 'protective coatings' in paints and coating nomenclature. The name 'fusion-bond epoxy' is derived from the way of resin cross-linking and their method of application which is different from that of a conventional liquid paint. FBE coatings are in the form of dry powder at normal atmospheric temperatures. The resin and hardener parts in the dry powder remain unreacted at normal storage conditions. At typical coating application temperatures, usually in the range of 180 to 250 °C (360 to 480 °F), the contents of the powder melt and transform to a liquid form. The liquid FBE film "wets and flows on to" 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", which means once the curing takes place, the coating cannot be converted back into its original form by any means. Application of further heating will not “melt” the coating and thus it is known as a “thermoset” coating. The world's leading FBE manufacturers are Jotun Powder Coatings, 3M, DuPont, Akzo Nobel, BASF and Rohm & Haas.
Since their introduction as a protective coating in early 1960ss, FBE coating formulations had gone through vast improvements and developments. Today, various types of FBE coatings, which are tailor made to meet the varying requirements of the industry are available in the market. Following recent research works by various FBE manufacturers, FBEs are available as stand-alone coatings as well as a part in multi-layers. FBE coatings with different chemical and physical properties are available to suit coating application on the main body of the pipes, internal surface, girth welds as well as on fittings. Similarly, variations are also available to match different ranges of pipeline service conditions.
Chemistry of FBE coatings
Essential components of a powder coating are
The resin and hardener part together is known as the "Binder". As the name indicates, in Fusion bonded epoxy coatings the resin part is an "epoxy" type resin. “Epoxy” or “Oxirane” structure contains a three membered cyclic ring — one oxygen atom connected to two carbon atoms - in the resin molecule. This part is the most reactive group in the epoxy resins. Most commonly used FBE resins are derivatives of bisphenol A and epichlorohydrin. However, other types of resins (for example Bisphenol F type) are also commonly used in FBE formulations to achieve various properties, combinations or additions. Resins are also available in various molecular lengths, to provide unique properties to the final coating.
The second most important part of FBE coatings is the curing agent or hardener. Curing agents react either with the epoxy ring or with the hydroxyl groups, along the epoxy molecular chain. Various types of curing agents, used in FBE manufacture, include dicyandiamide, aromatic amines, aliphatic diamines, etc. The selected curing agent determines the nature of the final FBE product — its cross linking density, chemical resistance, brittleness, flexibility etc. The ratio of epoxy resins and curing agents in a formulation is determined by their relative equivalent weights.
In addition to these two major components, FBE coatings include fillers, pigments, extenders and various additives, to provide desired properties. These components control characteristics such as permeability, hardness, colour, thickness, gouge resistance etc. All of these components are normally dry solids, even though small quantities of liquid additives may be used in some FBE formulations. If used, these liquid components are sprayed into the formulation mix during pre-blending in the manufacturing process.
FBE powder manufacturing process
Essential parts of a powder coating manufacturing plant are:
The components of the FBE formulation are accurately weighed and are pre-blended in high speed mixers. The mix is then transferred to a high-shear extruder. FBE extruders incorporate a single or dual screw setup, rotating within a fixed clamshell barrel. A temperature gradient[vague] between 50 °C and 100 °C is used within the extruder barrel. This setup compresses the FBE blend, while heating and melting it to a semi-liquid form. During this process, the ingredients of the molten mix get dispersed thoroughly. Because of the fast operation of the extruder and relatively low temperature within the barrel, the epoxy and hardener components will not undergo a significant chemical reaction. The molten extrudate then passes between cold-rollers and becomes a solid, but highly brittle sheet. It further moves to a “Kibbler”, which chops it to smaller chips. These chips are ground, using high speed grinders (classifiers) to a particle size of less than 150 micrometers (standard specifications requires 100% pass through in 250 micrometer sieves and maximum 3% retains in 150 micrometer sieve). The final product is packaged in closed containers — particular care is given to avoid moisture ingression. Normal storage temperatures of FBE powder coatings are below 25 °C (7 °F) in air-conditioned warehouses.
FBE coating application process
Regardless of the shape and type of steel surface to be coated, the FBE powder coating application has three essential stages:
The advantage of pipe and rebar is that their round shape allows continuous linear application over the exterior surface, while the parts are moved in a linear conveyor through the powder application booth. This method ensures high production rates. On fittings, etc., the coating is applied by manual spraying guns. Another method of application is "fluid-dip" process, in which the heated components are dipped in a fluidized powder bed (see below).
Surface preparation — blast cleaning
“Blast cleaning” is the most commonly used method for preparation of steel surfaces. This method removes rust, scale, slats, etc., from the metal surface in an effective manner and produces an industrial grade cleaning and a rough surface finish. The roughness of the steel achieved after blasting is referred to as profile, which is measured in micrometers or mils. Commonly used to profile ranges for FBE coatings are 37 to 100 micrometers (1.5 to 4 mils). Profile increases the effective surface area of the steel. The cleanliness achieved is assessed as NACE grades, or in accordance with Swedish standard (SIS) terminology of white-metal, near white-metal, etc.
It is very 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, compressed air (90 to 110 psi/610 to 760 kPa) is used to force an abrasive onto the surface, to be cleaned. Steel grit, steel shot, garnet, coal slag, etc., are the frequently used abrasives. Another method of blast cleaning is centrifugal blast cleaning, which is especially used in cleaning the exterior of the pipes. In this method, abrasive is thrown to the rotating pipe body, using a specially designed wheel, which is rotated at high speed, while the abrasive is fed from the centre of the wheel.
Heating and FBE powder application
Heating can be achieved by using several methods, but the most commonly used ones are "induction heating" or "oven heating". 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. Other methods of heating are "oven heating", "infra-red heating", etc. 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 "grounded" through the conveyor. The charged powder particles uniform wraps around the substrate, and melts into a liquid form. Internal surfaces of pipes are coated using spraying lances, which travel from one end to the other end of the heated pipe at a uniform speed, while the pipe is being rotated in its longitudinal axis.
Standard coating thickness range of stand-alone FBE coatings is between 250 and 500 micrometers (10 to 20 mils), even though lower or higher thickness ranges might be specified, depending on service conditions. 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 either by the residual heat on the steel, or by the help of additional heating sources. Depending on the FBE coating system, full cure can be achieved in less than one minute to few minutes in case of long cure FBEs, which are used for internal pipe coating applications.
Rebars are coated in a similar manner as coating application, on the exterior of pipes. For FBE coating application on the interior of pipe surface, a lance is used. The lance enters into the pre-heated pipe, and starts spraying the powder from the opposite end, while the pipe is being rotated on its axis and the lance pulls out at a pre-determined speed.
On fittings such as tees, elbows, bends, etc., powder can be sprayed using hand held spray guns. Small sized fittings can also be coated by dipping in a fluidized bed of powder, after heating the steel to the required powder application temperature. After field welding of the pipe ends, FBE can be applied on the weld area as well.
Advantages of FBE application over conventional liquid coating application are:
• ease of application,
• less waste of material,
• rapid application,
• and cure schedules, which means faster production rates.
• Finished coated pieces can be moved to the storage area within minutes after the application.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Fusion_bonded_epoxy_coating". A list of authors is available in Wikipedia.|