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Decoding Coil Coatings: Basics to Applications

SpecialChem – Sep 13, 2018

TAGS:  Industrial Coatings    

Efficient Method to Coat Metal - Coil Coating We would like to acknowledge ECCA for providing supporting information needed to develop this article

The coil coating process has been around for several years now! It allows companies to produce durable and attractive products.

Following its slogan “Finish first – fabricate later!”, the process enables manufacturers a way to paint the metal while it is still in sheet form.

The first coil coating line was installed in mid to late 1930’s by Joseph Hunter, the founder of Hunter Engineering. He used this method to paint venetian blinds as part of a continuous strip. After that, Hunter Engineering went on to design and build additional coil coating lines. It was eventually acquired by FATA S.p.a, the Italian global engineering company that is credited with building coating lines throughout the world.

Today, coil coating process presents several opportunities for the world's coating companies. The coil coating process is like those giant printing presses that produce the evening newspaper. The process doesn't stop, it runs at a constant speed and coils of metal are continuously spliced together so that the moving web is of unlimited length.


Let’s understand each step in this process in detail, but before that what exactly is coil coating process... Check out…


What is Coil Coatings Process?


According to EN-10169:2012-06, the coil coating process is a method of applying an organic coating material on rolled metal strip substrate in a continuous process. This process includes:

  • Unwinding the coil and cleaning, if necessary
  • Chemical pre-treatment of metal surface (either one side or two side), and
  • Single or multiple application of (liquid) paints or coating powders which are subsequently cured or laminated with plastic films
  • Cooling and rewinding the same coil for shipment to a sheeter, a slitter or a fabricator

All coil coating lines from the oldest and most basic line to the newest and most modern line have a number of common steps or processes.

A basic principle of coil coated material is that: the material travels through the various process sections in a
continuous manner without ever stopping or changing speed!

As a result, the material being coated has extremely consistent properties.

Besides the obvious labor efficiencies, environmental benefits and energy savings; coil coated material is inherently more consistent than those products produced on a part by part basis.

Coil Coating Process 
Source: NCCA


The coated surface can have a wide range of colors, gloss levels and surface textures. The choice of coating depends on the end use if the finished product such as coating with non-stick properties, great flexibility or high chemical resistance.


Substrates in Coil Coating Line – Entry to Exit


A typical organic coil coating line consists of decoilers, entry strip accumulator, cleaning, chemical pretreatment, primer coat application, curing, final coat application, curing, exit accumulator and recoilers.

Coil-coating lines vary greatly in size, with rated outputs between:
  • 3,000 tons per annum and 200,000 tons per annum of coated metal,
  • maximum widths from 150mm to over 2500mm and
  • speeds from 20 meters per minute to 200 meters per minute

Parameter  Value 
Operating speeds (m/min)  20 - 200 
Gauge (mm)  0.1 - 3.0 
Width (mm)  150 - > 2500 
Coating thickness (microns, each side)  2 - 200 


Continuous Operations on Coil Coating Line - Accumulators


This basic principle of continuous process is achieved through the use of entry and exit accumulators.

Entry Accumulator – It allows an operator to stop the entry end and splice in another coil while the coil being processed continues to run through the line unaffected by the activities at the entry end.

When the entry end is stopped, this accumulator continues to feed material into the coil line as the upper and lower accumulator rolls converge. When the splice is completed, the entry end is restarted, the accumulator rolls diverge, and the entire line is once again in a steady state.

This ability to continuously splice coils together without stopping the entire line creates an unbroken web that is virtually unlimited in length. This process repeats itself until the last coil of metal is processed.

Exit Accumulator – It performs an opposite function from that of the entry accumulator. While an entry accumulator feeds material into the line as the entry end is stopped, the exit accumulator absorbs material from the line. This allows the exit end of the line to stop without affecting the process.

In this case, the upper and lower rolls diverge as more material is put into the accumulator while the exit end operator removes the completed coil and readies the exit end to rewind yet another coated coil of metal.

Most of the newer lines designed today have smaller exit end accumulators in that there have been several advances in the technology of cutting the coil, removing scrap and rewinding a bare mandrel. These more time efficient processes have resulted in the need for less metal accumulation at the exit end.

In this way, the critical coating operations in the process section operate at a
constant speed to ensure high quality and consistency!

Accumulators can be either vertical or horizontal. They can be activated through extremely long hydraulic cylinders or mechanical winches. Some accumulators can accommodate as much as 650 feet of material while others hold less than 100 feet of material. Their size is determined by the maximum line speed of the coil line and the amount of time an operator needs to feed and splice additional coils.


Entry, Cleaning and Pretreatment


ENTRY: Bare coils of metal are placed on an unwinder or decoiler. The front end of the coil fed up to a joiner, which will link this to the back end of the previous coil (coil splicing)

Joining is normally done by mechanical means, where the coil ends are sheared to provide a straight edge and then ‘stitched’ together. As a simple process, it is both low cost and low maintenance and provides a strong bond.

Metal Coils Entry in Coil Coating Process
Whilst most coil coating lines use a mechanical stitch to join coils, there are alternatives:

  • Adhesive or adhesive tape can be used to stick one end of the coil to the other
  • Welding is commonly used on other types of continuous processing lines, such as high-speed annealing and galvanizing lines.

As soon as the joint is made, the coil is fed into the accumulator at a much faster speed than the process section is removing it, building up a buffer of material in the accumulator.

CLEANING: Before applying any coating, it is essential that the surface of the coil is free from impurities such as grease, oil, carbon or abraded metal particles. Cleaning can consist of more than one stage and in some cases includes a rotary abrasive brush section to remove any localized corrosion products from the substrate.

  • For steel and zinc coated steel substrates, alkaline cleaners are invariably used which are the most efficient for removing organic soils
  • Also, alkali cleaning is commonly used on aluminum surfaces but here, acidic cleaning can also be utilized
  • Electrolytic cleaning is occasionally used on lines that process aluminum

PRETREATMENT: The pretreatment of the substrate has the role of providing compatibility between the metallic substrate and the applied organic coating to ensure the optimum adhesion and corrosion resistance characteristics. In order to ensure organic coatings adhere properly to metallic surfaces, a chemical conversion or pre-treatment must be applied.

Pre-treatment solutions are often applied by:
  • Spray or with a simple roller-coater, called a chemical coater (chemcoater)
  • Immersion, which require subsequent rinsing

The chemcoater contains a roller for the top and bottom surface of the coil that takes the solution from a tray and applies it to the coil surface. By controlling the speed of the rolls and the gap (or pressure) between this applicator roll and the coil, the amount of coating applied can be controlled.

All applied pretreatments require to be dried using a low temperature oven or by relying on the heat in the metal substrate.

Conventional conversion coatings include iron phosphate, zinc phosphate, alkaline oxide and, for special applications, chromates. No-rinse processes are based on chromates or modern chrome-free chemistry. The no-rinse systems have the environmental benefits of no polluting effluent and plant requirements are reduced, thus saving on energy and capital investment.

The two most popular substrates are hot dip galvanize (HDG) and galvalume (GLUM) and both continue to demonstrate superior performance when treated with conversion treatments.


Paint/Coating Application


Paint/Coating Application in Coil Coating Process After pre-treatment process, the painting process comprises of two stages, namely:

  • Primer application and
  • Finish coat application

…where coating is applied in substrate using roller coater.

There are several general classes of coil coating lines.

  1. One class has a single coater and oven
    These lines serve markets which are predominately consumer products orientated and have products constructed with cold rolled steel (CRS), tin mill black plate (TMBP) and/or tin free steel (TFS). Such products as curtain rods, shelving, small appliance wrappers and hot water heaters are found in these markets. Although limited in their product offering, these lines enjoy a substantial cost advantage over lines that operate two or more coaters and ovens in a single line.

  2. The lines with two or more coaters are called tandem lines
    They have the ability to apply a primer and a top coat in a single pass. These lines predominately serve the construction and appliance markets.

  3. There also exists several lines that have three coaters and three ovens
    They serve a highly specialized market for construction products that require a primer, a top coat and a barrier coat. As might be expected, these products are extremely weather resistant and are generally found in highly corrosive environments.

In general, roller coaters are automated machines that coat one or both sides of a flat substrate with precisely controlled thicknesses of coating material. Many types of formulations can be applied with roller coating but must have good flow properties (to avoid ribbing), flexibility, adhesion, and high opacity. Low surface tension is also necessary for wetting the rolls.

  • In the first stage, the metal strip passes through a roller coater machine that applies a primer paint coat to both sides of the strip
  • In the second step, once primer is dried, the strip passes through a second roller coater machine which applies the finish paint coat to either one or both surfaces.

There are a number of coater designs depending on the configuration of the coil line, the types of coatings being used, and the types of metal being coated. In almost every case however, the coater is equipped with:

  • A pan,
  • A steel or ceramic pick up roll, and
  • A rubber covered coating roll

Reverse Roll Coating Process:
  • The coating is circulated in the paint pan
  • A rotating pick up roll is partially immersed in the coating and rotates in a counter clockwise direction
  • The coating is then transferred onto the coating roll which is rotating in a clockwise direction

This particular configuration is used for the majority of most products and is quite capable of achieving coating thicknesses of 1.0 mils (.001") with most coatings.

Reverse Coil Coating
Reverse Coil Coating
Direct Coil Coating
Direct Coil Coating

Direct Coating Process:

Thinner coatings and printing use a similar configuration, but the roll directions are reversed so that the coating roll has the same surface direction as the web and the pick-up roll has the same surface direction as the coating roll. Direct coating requires additional equipment because of the dangerous pinch point between the two rolls.

Three Roll Coating Process:

A third configuration is also possible if the coater is equipped for an additional roll. Three roll coating is generally used for those coatings requiring additional thickness such as PVC's and coatings which are being applied as a stripe with bare edges. This is fairly common when producing material for garage doors and lighting fixtures. Three roll coating uses a third roll positioned behind the pick-up roll and the rotational direction of all three rolls is clockwise.

In almost all the configurations, the wet film is controlled by:

  • The rotational speed of the pick up roll
  • The rotational speed of the coating roll and
  • The gap between the pick up roll and the coating roll

The gap between the coating roll and the web has little effect on the wet film thickness being applied and is adjusted more for cosmetic reasons and to accommodate different surface textures of the metal being coated.

Some coil coating lines can include the application of a laminated film using rolls of thin polymer film. Glue is applied to the metal surface immediately before laminating, or a paint layer such as a primer is used to fulfil the same purpose. The laminate is bonded directly and limited exposure to heat in the ovens may be needed to completely cure the adhesive layer.

This is generally rolled under pressure onto the strip after the top coat oven which is used for curing an adhesive which is applied by the top-coat roller coater.

The two most common coater designs are the "S Wrap" coater and the horizontal coater.

The "S-Wrap" coater is the more versatile of the two and has proven more capable of coating a variety of different coatings and metal shapes  The horizontal coater is more typically found as a treatment coater but is also found at the end of some lines coating either lubricants or waxes. 
 S-Wrap Coater
S-Wrap Coater
 Horizontal Coater
Horizontal Coater


Curing of Paint


There has been as many variations and advances in the curing section of coil coating lines than probably exists in all the other sections combined. Although the ovens may look the same from the outside, there are substantial differences that have a dramatic effect on the actual process of removing solvents and curing the coating.

After each step (primer as well as final coat application), the strip passes through an oven to ensure that all the volatiles are removed and that the paint is fully cured.

Methods that can be applied to cure paints include:
  1. The most common drying and curing technique is to use gas-fired ovens for either convection or flotation curing.

    Although the natural gas convection oven remains the standard of the industry, the ability to continuously monitor solvent concentrations with reliability has only recently occurred. This ability has spawned the use of variable exhaust rates which have dramatically improved energy efficiencies.

    High impingement floatation ovens have increased line speeds because of better energy transfers. This has been especially evident with the coaters who produce high temperature bakeware coatings that require temperatures that are almost twice the curing temperatures of polyesters and acrylics.

  2. There are two types of electrical curing technique which are in use for coil coating, namely induction and IR (or NIR) curing.

    Induction curing works by heating the metal substrate by passing the metal strip through a magnetic field which induces electrical currents in the strip and, through resistance, a heating effect. While IR and NIR curing only heats the coating, not the metal substrate or surrounding air.

    Coaters who use water base coatings are able to take advantage of electrical and gas fired IR ovens as they reduce their maintenance costs. Electrical induction coils which were once used only on galvanizing lines are now being used as both the primary and secondary curing sources with excellent results.

  3. Radiation cured coatings are widely used in many different industries, but as yet have not found significant use in coil coating. Radiation curing uses electron beam (EB) or ultraviolet (UV) light to polymerize a reactive and usually solvent-free coating material.

    There is significant development work ongoing on this to achieve the same properties from radiation cured coatings as from conventional coil coated products.

Also Read: Hybrid Cure Process for Coatings: Achieve Desired Finish Efficiently

After curing, the metal strip is then water quenched and air-dried. For the production of plastisol products, an embosser roll may be used to imprint a pattern into the paint surface prior to quenching.


Exit Accumulator and Rewinding


Once the paint film is fully cured and quenched, the strip then passes through the exit accumulator, again to support the continuous process, before being sheared and recoiled to the customer’s dimensions. A flying shear is commonly used, allowing the removal of uncoated material at coil ends or defective material whilst the line is still running at high speed.

The strip is inspected and tested to ensure that it complies with quality standards and coiled onto a tension reel.


Key Coatings Types Used in Coil Coating Process


Two key types of coatings used for coil coating include:
  • Liquid paints
  • Powder coatings

…offering different levels of durability and performance, and to satisfy different aesthetic requirements.

Also, instead of lacquers or additional to them films are used where very high aesthetic quality is required. Films are supplied in a variety of film thicknesses, colors and finishes (smooth, structured or printed).


Liquid Paints


Liquid paints must satisfy a number of fundamental requirements in order to be successfully applied:
  • Easy application to give uniform appearance and defect-free films even on the fastest production lines (up to 220m/min)
  • A very short cure time during which the pigment colours remain stable – time spent in the oven can vary between 6 to 60 seconds depending on the line while the substrate temperature is between 210ºC and 250ºC according to the paint used
  • Flexibility to enable the coated metal to be bent without cracking or loss of adhesion of the paint film

The dry film thickness of the coatings is usually from 5 to 35 microns for a primer and 15 to 200 microns for topcoats. Thickness is determined by the paint qualities chosen and the final requirements of the prepainted metal product.

There are two types of paint used for coil coating, based on either thermosetting or thermoplastic resins. The resin types can be subdivided into more specific categories: alkyds, acrylics, epoxies, polyesters, polyurethane, silicone-modified resins, polyvinyl chlorides and polyvinylidene difluorides.

Types of Coating  System Exterior Performance  Color Performance  Gloss Performance  Flexibility  Most Suitable Markets 
Acrylics  Good  Good  Good  Fair to Good  Construction, Transportation 
Polyesters  Fair to Good  Good  Good  Very Good  Transportation, Consumer Business 
Silicone Polyester  Very Good  Very Good  Very Good  Good  Construction 
PVC/Plastisols  Excellent  Good  Good  Very Good  Construction, Transportation 
Fluorocarbons  Excellent  Excellent  Excellent  Excellent  Construction 

Coatings/Markets Reference Chart

  • Alkyds, solution vinyls and to some extent, epoxies enjoyed substantial market share during the early days of coil coating
  • Solution vinyls were especially popular because of their excellent adhesion properties
  • Polyesters allowed the coaters to dramatically expand their pallet of color and gloss offerings
  • Siliconized polyesters greatly increase the long-term durability on exterior metal panels


Powder Coatings


Powder coatings can be described as “solid paint” which can be melted to form a continuous film over the substrate.

These coatings are easily applied by either electrostatic or tribostatic spray to cover the article being painted. As the coating thickness builds up, the coatings are self-limiting, so that an even film thickness can be achieved. The powder-coated article is then cured, when the powder melts and flows out to form a continuous paint film before reacting chemically to become a solid, inert coating.

  » Check Out Various Ingredients for Powder Coatings Used for Coil Coating Process


Lamination


Instead of lacquers or additional to them films are used where very high aesthetic quality is required.

There are 4 main film types:
  • Polyvinyl chloride (PVC)
  • Polyvinyl fluoride
  • Thermoplastic acrylics and
  • Polyethylene terephthalate (PET)

Their advantages include high flexibility and suitability for deep drawing. Co-extruded qualities also tend to have very high gloss and good hardness.

Co-laminating - Co-laminating is a hybrid of coating and laminating, where liquid-applied coatings and laminates are combined. Typically, a liquid primer and/or colored basecoat are firstly applied, over which a clear laminate is added. The clear laminate can impart excellent scratch and abrasion resistance, together with attractive aesthetic properties making it suitable for high contact domestic appliance applications such as fridge-freezer doors.


Quality Control and Product Testing


Some of the most common methods used for the development of new products for coil coating process include:
  • Viscosity
  • Cure
  • Gloss and color
  • Pencil hardness
  • Flexibility

VISCOSITY: Depending on the type of paint, two different methods to measure the consistency of a coating can normally be used.
  • For systems such as polyester, polyurethane, PVdF and epoxy, the paint is poured into a container (cup) that has a small hole bored in the bottom. The time taken for the paint to empty from the cup is measured. The specification should state at what temperature the test is to be performed and what is the type of cup (BSB4, Ford 4, etc.)
  • For more viscous paint such as PVC plastisol, the Brookfield Rotating Viscometer is frequently used

CURE: A quick test for paint cure is performed by soaking a piece of cotton wool with a strong solvent such as MEK (methyl ethyl ketone, butan-1-one) and rubbing the paint to see if the solvent can penetrate it. Most topcoats that have been cured in a chemically cross-linking process will withstand at least one hundred rubs.

COLOR & GLOSS: These properties are checked with calibrated instruments. In both cases light is projected onto the surface. The reflected beam is collected and analyzed by the equipment. The angle of incident light should be quoted with a gloss reading. Color is measured by a colorimeter.

PENCIL HARDNESS: The hardness of the coating can be quoted from its resistance to being gouged by pencils of different hardness.

FLEXIBILITY: Three tests are generally performed for flexibility:
  • Firstly, the T-bend test (EN13523-7) where painted metal is bent back onto itself to see if cracking has occurred
  • The second method for checking flexibility is to make a slow draw indentation and observe for cracking. This is frequently referred to as the Erichsen test
  • The third method is rapid deformation also known as the reverse impact test (EN13523-5)



Main Applications and Key Benefits of Coil Coating Process


There are four major markets for coil coated material. Each market is predominately served by one or two specific resin types. For example, almost all appliance type products are produced with material coil coated with polyesters.

Markets  Applications 
Contruction
Construction 
  • Roofing
  • Exterior panels
  • Interior panels
  • Garage doors
  • Rainwear
  • Agriculture panels and components
  • Entry doors
  • Roof deck
 
Transportation
Transportation 
  • Brake components
  • Gas tanks
  • Engine components
  • Truck body panels
  • Tractor body panels
  • Bus body panels
  • RV panels
  • Automobile exterior & interior trim components
  • Accessories
Industrial Applications
Industrial Applications 
  • Machinery/Industrial equipment
  • Vending machines
  • Bakeware
  • HVAC (less baseoard heating)
  • Baseboard heaters
  • Lighting fixtures
  • Signs and displays
  • Swimming pools
 
Consumer Goods
Consumer Products 
  • Signs and displays
  • Computer components
  • Consumer electronics (less computer)
  • Shelving
  • Window accessories (curtain rods, blinds, muttons)
  • Furniture components (conduits, raceways, wall panels)
  • Signs and displays
  • Small appliance (less consumer electronics)
  • Major appliance/critical surface "White Goods"
 
Container Packaging
Container/Packaging 
  • Cans/Containers
  • Ends, tabs, closures
  • Crowns
  • Barrels, drums, pails
  • Strapping, seals
 

For the coil coating industry to prosper over the long term, it will continue to give advantages over competing industries in a number of areas:

  • Product Durability: some architectural products have up to a 40-year life before any repainting is required. Warranties are often issued by suppliers to reinforce their claims
  • Recyclability: prepainted metal is fully recyclable and a workable infrastructure exists to have scrap metal recycled
  • Low consumable materials consumption: the coil coating process embodies intrinsic efficiencies such as high coating transfer efficiency, and custom width substrate
  • Low emissions: low waste emissions including water and VOC's
  • Consistency of quality: coil coating is a highly automated continuous process that lends itself to the production of consistently high-quality products
  • Cost effectiveness: for most applications, the coil coating process represents the most cost-effective way to paint a metal surface
  • Safety: the highly automated and industrialized process is intrinsically safe


Current Challenges in Coil Coatings


Since the great recession of 2008, the coil coating industry has seen several mergers and acquisitions. This has resulted in few large coating companies in the marketplace. In spite of this, there remains an excess capacity for coil coating globally. This has resulted in razor-thin margins due to:
  • Increased competition and
  • Driven down prices

The increase in competition has made it necessary for coaters to improve their control strategies to enhance:
  • Speed
  • Accuracy
  • Quality
The main technical challenge is to ensure consistent quality (color match, film build…) & avoiding edge-to-edge variations in repeated runs.

The solution lies in proactively managing coatings viscosity to reduce rejects and rework/defects. By employing innovative techniques to control temperature & viscosity, you can preserve your cost margins, while meeting your customers’ increased demands! Want to know more?

Take this course by Michael R. Bonner and benefit from his 20+ years’ experience in the science of point-of-use temperature and viscosity control in fluid dispensing processes.

Coil Coatings: Reducing Surface Defects, Material Usage & Costs





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