Prepainted Steel Coil: Substrate, Coatings and Properties

Prepainted steel coils, also known as pre-coated steel, color-coated steel coil, or PPGI (pre-painted galvanized iron) coil, have been coated with primer and paint before they are cut, shaped, or formed into final products.

This process ensures a uniform paint application, contributing to the material’s aesthetic and functional properties.

The coils are commonly made from galvanized or Galvalume steel, which provides a rust-resistant foundation, and the pre-applied paint adds an additional layer of protection against environmental factors.

Sectional Structure of Prepainted Steel

To fully understand the knowledge of pre-painted steel coils, you must first learn its sectional structure.

The sectional structure of prepainted steel from the bottom to the top typically consists of the following layers:

• Base Metal (Substrate): The base metal is usually galvanized steel or galvalume steel. Galvanized steel has a coating of zinc for corrosion resistance, while galvalume steel has a coating made from aluminum, zinc, and silicon.

• Pretreatment Layer: Before applying the primer and paint, the steel is cleaned and treated with a pretreatment solution. This layer provides good adhesion for the primer and paint and also enhances the corrosion resistance of the material.

• Primer Coating: The primer is applied to the treated base metal to further improve corrosion resistance and ensure that the topcoat adheres properly. The primer layer also provides a uniform surface for the topcoat.

• Topcoat (Finish Coat): The topcoat is the visible layer and provides color and aesthetic appeal. It also adds additional protection against the elements. The topcoat can be chosen for specific properties, such as UV resistance, flexibility, and hardness, depending on the application.

• Backer Coat (Back Coat): On the reverse side of the steel, a backer coat is applied. This layer is usually a thinner layer of primer or a specific backer coat that protects the steel and can enhance the adhesion of adhesives if the steel is to be laminated or bonded to other materials.

• Protective Film (Optional): Some prepainted steel products may have a removable protective film applied to protect the finished surface during transportation and installation.

The specific materials and thickness of each layer can vary depending on the manufacturer’s specifications, the intended use of the product, and environmental considerations.

The paint system used for the topcoat can be a polyester, silicone-modified polyester (SMP), polyvinylidene fluoride (PVDF), or other types depending on the requirements for durability, color retention, and resistance to chalking.

The Substrate of Pre-Painted Steel Coil

The substrate of a pre-painted steel coil refers to the base metal onto which the paint and other coatings are applied during the coil coating process.

The choice of substrate is critical because it must possess the necessary mechanical properties and corrosion resistance for the intended end use.

The most common substrates used for pre-painted steel coils are:

• Hot-Dip Galvanized Steel (GI): This substrate is steel that has been coated with a thin layer of zinc to provide corrosion resistance. The zinc coating is applied through a hot-dip process, where the steel is submerged in a bath of molten zinc. The zinc layer protects the steel by acting as a barrier to corrosive elements and by providing sacrificial protection where the coating is damaged.

• Galvalume Steel (GL): Galvalume is a coating made of a combination of aluminum, zinc, and a trace amount of silicon, which is applied to steel. The presence of aluminum in the coating provides excellent protection against corrosion, especially in more extreme environments, and the zinc offers additional protection by galvanic action.

• Electro-Galvanized Steel (EG): This type of steel is galvanized using an electrochemical process, which provides a zinc coating that is uniform in thickness and has a smooth finish. Electro-galvanized steel is commonly used where precise and aesthetic applications are needed.

Types of Coatings

There are several types of paints and coatings used for prepainted steel coils, each with its own set of properties and applications:

• Polyester (PE): Polyester coatings are the most common type for prepainted steel coils. They offer good adhesion, color retention, and flexibility. They are suitable for general use and are cost-effective, making them a popular choice for buildings, roofing, and wall panels.

• Silicone Modified Polyester (SMP): SMP coatings have a silicone resin added to the polyester coating, which provides better UV resistance and maintains color and gloss for a longer period compared to standard polyester. They are used for applications that require greater durability and exposure to harsh weather conditions.

• Polyvinylidene Fluoride (PVDF): PVDF coatings are high-performance coatings that provide excellent resistance to fading, chalking, and chemical degradation. They are highly durable and are used in applications that require long-lasting protection, such as commercial and residential buildings, and architectural projects.

• Epoxy: Epoxy coatings are known for their superior adhesion and corrosion resistance. They are typically used as a primer layer beneath other types of coatings to enhance the overall protection of the steel coil.

When selecting a paint type for prepainted steel coils, it’s important to consider the environmental conditions the material will be exposed to, the desired aesthetic qualities, and the longevity required for the application.

The choice of coating will directly impact the performance and lifespan of the finished product.

Below is a table comparing some of their key properties and typical applications:

Life of Prepainted Steel Coil

Paints provide additional protection to metallic-coated steel by acting as a barrier against moisture, oxygen, and other corrosion-inducing agents, and by containing specific corrosion-inhibiting agents. 

Factors affecting the life of prepainted steel coil include paint thickness,  type of paint, type of pre-treatment, primer coatings thickness, and application environment. 

The lifespan of a paint coating can be measured by different criteria, such as noticeable fading, adhesion loss, or steel corrosion. Factors like UV stability and resistance to environmental chemicals are crucial. 

Corrosion failures involve significant metal loss, and maintaining paint integrity is crucial. For example, a high-quality paint system can extend the lifespan of a G90 galvanized coating on a roof from 15-25 years to 40-50 years or more in a moderately corrosive environment.

Additionally, repainting upon initial signs of corrosion can add another 10-15 years to the roof’s life.

Source: https://www.galvinfo.com/wp-content/uploads/sites/8/2017/05/GalvInfoNote-4-1.pdf

Specification

Uses

Here are some common applications of pre-painted steel coils:

Construction Industry:

   – Roofing and wall panels for commercial, industrial, and residential buildings.

   – Garage doors and shutters.

   – Insulated panels for energy-efficient buildings.

   – Gutter systems and downspouts.

   – Ceiling systems and interior partitions.

Home Appliances:

   – Refrigerators, washers, and dryers.

   – Air conditioning units.

   – Microwave ovens and other small appliances.

   – Water heaters.

Properties of Pre-Painted Steel Coils

Pre-painted steel coils are designed to offer enhanced corrosion resistance through a combination of protective layers and chemical treatments. The principles that contribute to the corrosion resistance of pre-painted steel coils include:

• Barrier Protection: The paint layers on pre-painted steel act as a barrier, protecting the underlying metal from environmental elements such as water, oxygen, and contaminants that can cause corrosion. By preventing these elements from reaching the metal surface, the paint coating significantly reduces the risk of rust and corrosion.

• Adhesion: Proper adhesion of the paint to the metal surface is critical for corrosion resistance. Good adhesion prevents moisture and corrosive substances from penetrating the coating and reaching the steel substrate. The pretreatment process, which typically includes cleaning and applying a conversion coating such as phosphate or chromate, enhances the adhesion of the paint layers.

• Chemical Inhibition: Some pretreatment coatings contain corrosion inhibitors that actively prevent corrosion through chemical reactions. These inhibitors can neutralize corrosive substances or passivate the metal surface, making it less reactive and less prone to corroding.

• Cathodic Protection: In some cases, the primer or pretreatment layer may contain sacrificial elements like zinc (in the form of zinc phosphate or galvanized coatings). These elements are more reactive than steel and corrode preferentially, protecting the steel by sacrificial action. This is known as cathodic protection.

• Edge Protection: The edges of steel coils are particularly vulnerable to corrosion since they may have a thinner coating and are more exposed to the environment. Special attention is given during the coating process to ensure that edges are adequately coated. In some cases, additional edge sealants may be applied for enhanced protection.

• UV Resistance: Ultraviolet (UV) light can degrade some coatings over time, leading to reduced corrosion resistance. The topcoat on pre-painted steel often includes UV-resistant chemicals that protect the underlying layers from UV-induced degradation, thereby maintaining the integrity of the protective coating.

By incorporating these principles, pre-painted steel coils are able to offer long-lasting durability and effectiveness in environments where corrosion.

Process of Pre-Painted Steel Coil

The fabrication of prepainted steel coils and sheets begins with the delivery of the substrate in coil form directly from the rolling mills. There are three sections involved in the coil coating line: entry, processing, and exit.

Each step ensures that the finished prepainted metal products are durable, high-performing, and aesthetically appealing. The machines in the production line work simultaneously to paint 2,000 to 4,000 square feet of metal strips per 4 minutes.

Here are steps at the coating line:

Step 1: Cleaning the Metal Strip

Cleaning up the oil, dirt, and impurities of the cold-rolled coil involves brushing, an alkaline dip, a water bath, or brushing.

Step 2: Application of Zinc and Alloy

The application of aluminum and zinc depends on the application and customer requirements. Some products consist of 100 percent zinc, while others contain magnesium alloy.

Step 3: Treatment of the Surface

This step is vital to ensuring that the paint and primer bond well with the metal strip. Due to some chemical restrictions, the surface is treated with chemicals, such as alkaline oxidation, depending on the supplier and country of origin.

Step 4: Metal Rinsing and Drying

Rinsing the metal strip helps ensure that it has no chemical residue before applying primer. Drying the metal is a must to ensure that the subsequent layers bond well.

Step 5: Application of Primer

A coating of primer can be on one or both sides of the metal strip. Primer application hastens the adhesion of the finished paint layer.

Step 6: Curing and Cooling the Strip

Curing takes 15 to 16 seconds. This method allows the primer to harden, dry, and reach the desired temperature before applying the next coating.

Step 7: Front and Back Coating

This step is the final painting stage, wherein two-coat paint systems are applied.

Step 8: Drying, Curing, and Cooling

The product undergoes this step to ensure that the steel product is ready for rewinding, rerolling, and shipping to end users.

Step 9: Packaging

Image showing a coil coating line schematic.

Source: https://www.coilcoating.org/process

Conclusion

Pre-painted steel coils provide durability and longevity, ensuring structures benefit from their resilience against environmental factors.

Their aesthetic versatility allows for a wide range of color options, while the uniform application enhances overall visual appeal.

The production process is efficient, resulting in time and cost savings for manufacturers and end-users alike.