Exterior Service Finish Coat Performance: A Comparison of Acrylic Polyurethane, Polyester Polyurethane, Polyaspartic, Fluorourethane and Polysiloxane Coatings

Introduction

For decades, traditional aliphatic acrylic polyurethane topcoats were the mainstay as finish coats for exterior environments where resistance to solar radiation (sunlight) was desired to preserve color and gloss and provide corrosion and chemical resistance. However, ultraviolet light degradation of the urethane resin would cause a color shift (fading) and gloss reduction of the finish coat (particularly with darker colors) well before the underlying coating layers (e.g., epoxy mid-coat and/or zinc-rich primer) showed signs of deterioration. Without the ability to selectively remove and replace the finish coat while preserving the underlying layer(s), removal and replacement of the entire system was common practice, effectively “throwing the baby out with the bath water.” In the ideal world we would formulate the finish coat to last as long as the underlying layers.

Finish coat formulations have entered the marketplace within the past decade or so that are touted to, in some cases, outperform traditional acrylic polyurethane coatings. This brief article explores the advantages and limitations of various finish coat formulations.

Background

Polyurethane coatings are a versatile coating type that have been used for many applications. Varying the chemistry and functional groups in these coatings can result in a difference in physical properties, making them suitable for different service environments. Typically, polyurethanes are specified for use when good weatherability (color and gloss retention) and corrosion resistance properties are desired. This article focuses on four variations of polyurethane coatings and one non-urethane formulation that provides similar properties.

General Chemistry of Polyurethanes

Strictly speaking, to be considered a polyurethane coating, an isocyanate group must be reacted with an alcohol to form the urethane functional group as shown in Figure 1:

Polyurethanes are created by reacting polyisocyanates (multiple functional isocyanate groups) with polyols (multiple functional alcohol groups). The polyisocyanates can be aromatic (containing a disubstituted ring) or aliphatic (the atoms are arranged in open chains). Aromatic polyurethanes are subject to yellowing when exposed to solar radiation (sunlight), whereas aliphatic polyurethanes have much better color and gloss retention and weatherability.

Health Risks of Isocyanate

An isocyanate group consists of one nitrogen atom, one carbon atom, and one oxygen atom each joined by double bonds. Exposure to isocyanates can result in irritation or sensitization. Irritation can occur on the skin, in the respiratory tract or eyes and present as redness, itching, rash and coughing. Sensitization is more like an allergic reaction that can manifest on the skin or in the respiratory tract causing more severe irritation. It is important to practice proper chemical hygiene and respiratory protection when working with isocyanate containing materials. This information is available on the Safety Data Sheet (SDS) produced by the coating manufacturer.

Polyacrylate (Acrylic) Polyurethane

Formulation

In the case of an acrylic polyurethane, the polyol component is a polyacrylate, or acrylic. The chemical structure of methyl methacrylate is provided in Figure 2. Some of these can be styrenated, which introduces an aromatic ring on the polyol component. Adding styrene to the formulation can increase flexibility characteristics, as well as chemical and water resistance; however, it reduces the weatherability characteristics.

Advantages

• Excellent weatherability (unless modified with styrene)
• Styrenated: Increased chemical and water resistance
• Relatively low cost
• Dries quickly
• Scratch resistant

Limitations

• Styrenated: Reduced weatherability
• Contains isocyanate (potential health risk)
• Adhesion (relative to polyester polyurethanes)
• Abrasion resistance

Uses

Like many polyurethane coatings, acrylic polyurethanes are well suited to exterior steel surfaces due to their excellent weatherability. However, many acrylic polyurethanes are used on wood surfaces such as tables or cabinetry. The high scratch resistance makes this a great application for furniture or decorative items, but the poor abrasion resistance makes these coatings unsuitable for flooring applications.

Polyester Polyurethane

Formulation

The polyester polyurethane chemistry is much the same as the acrylic polyurethane, but the polyol component consists of polyester resin. An ester functional group contains an oxygen atom double bonded to a carbon atom, which has another oxygen atom sandwiched between two carbon atoms, see Figure 3.

Advantages

• Enhanced chemical resistance
• Good flexibility (increased impact resistance, tensile strength and tear resistance)
• Good abrasion resistance
• Relatively low cost

Limitations

• Higher viscosity
• Contains isocyanate (potential health risk)

Uses

Due to the good impact and abrasion resistance, polyester polyurethanes make excellent topcoats for flooring systems. The enhanced chemical resistance properties makes them a good consideration for industrial environments where acids, oils or other materials may come in contact with the coated surfaces.

Polysiloxane

Formulation

While polysiloxane coatings are widely used as finish coats for exterior surfaces, they are not based on urethane chemistry. Rather, a siloxane contains repeating silicon-oxygen groups in the backbone, side chains, or crosslinks. The most common organic polysiloxanes are formulated with epoxy or acrylic resin.

Advantages

• Rapid curing under correct conditions
• Reported infinite recoat-ability (manufacturer-specific)
• Good flexibility
• Resistant to solar radiation
• Hard film formation
• Good cleanability
• Increased heat resistance
• High-solids (Low VOC content)
• No isocyanates to create a health risk

Limitations

• Relative humidity of 50%+ typically required for curing
• Brittle at high coating thickness
• Higher in cost

Uses

Polysiloxane coatings can be used as a topcoat in a three coat system, or they can be applied as a high build finish coat over a zinc-rich primer in a two-coat system.

Polyaspartic

Formulation

Conventional two component aliphatic polyurethane coatings are modified with aspartic ester resin.

Advantages

• Faster dry times
• Higher film build
• Good weathering characteristics
• Good abrasion resistance

Limitations

• Poor acid resistance
• Short pot life may mean smaller batch sizes or specialized equipment
• Potentially short recoat interval
• Variable cure time dependent on prevailing ambient conditions

Uses

The higher film build allows this coating to provide more coverage in one coat. They can be used as a topcoat in two-coat systems or even as a direct to metal (DTM) in less corrosive environments. The fast-dry time along with the reduction of necessary coating layers makes this an ideal coating for projects that have a need for fast return to service. The use of a polyaspartic system can reduce painting schedules to one shift as opposed to several days to allow for proper cure of each coating layer.

Fluorourethane

Formulation

Fluorourethane coatings are more commonly referred to as fluoroethylene vinyl ether (FEVE) coatings. FEVE resins react with conventional aliphatic isocyanate hardeners, giving them their urethane functionality.

Advantages

• Excellent weatherability
• Excellent corrosion resistance
• Chemical and solvent resistance
• Color and gloss retention exceeding 50 years with little change

Limitations

• High cost
• Limited pot life
• Contains isocyanate (health risk)
• Presence of fluorinated material may cause environmental concerns

Uses

The use of FEVE topcoats is more prevalent outside the US; these finish coats are required to be used on large steel bridges in Japan due to their long-term weatherability. While the material costs are relatively high, use of these topcoats reduces maintenance painting costs and the overall life cycle costs.

Summary

Exterior service finish coats are the first line of defense against corrosion and the effects of weathering. In many cases aesthetics is a primary consideration, even though the finish coats are being applied to industrial structures. For decades acrylic polyurethane products have been used as cost-effective topcoats to provide good durability and weatherability. The desire to perform less frequent maintenance painting while preserving aesthetics like color and gloss have created a market for alternative technology topcoats. This article has described the chemistry, uses, advantages and limitations of four alternative finish coats that can, in some cases provide a longer service life. The key is to assess whether the increase in upfront material costs can be recovered by lowering life cycle costs.

Posted by KTA’s Valerie Sherbondy