Several methods are used to protect metallic components from corrosion due to severe environmental conditions, such as the effects of high humidity, seawater, high temperature, and chemical attacks, among others. Within these methods, it is very common to use coating systems. This article will briefly summarize the differences between Polyurea and Polyurethane (protective coatings).
Chemistry of Polyurethanes and Polyureas
Protective coatings and linings are often made from polyurethanes or polyureas. Polyether polyamine is for polyureas, while polyetherpolyol is for polyurethane. Both systems are made up of two components, and the curing mechanism is by co-reaction. For polyurethanes, the isocyanate is the common component. It is also possible to make so-called hybrid systems in which the isocyanate is reacted with a mixture of hydroxyl and amino groups.
Figure 1. Figure 1.
Polyurethanes can be synthesized during the polyaddition reaction by using suitable catalyst systems to increase the reaction time of the components. There are two main types of catalysts: metal complexes and amine compounds. The selectivity of the targeted Isocyanate reactions is affected by choice of catalyst.
There are two types of polyurethane: aliphatic and aromatic. However, there are some differences. Due to the use of catalysts, the polyurethane reaction can be sensitive to low temperatures and moisture. Due to the competition between hydroxylpolyol, water, and an isocyanate group for the reaction, the polyurethane system can blister when humidity is higher than 5%.
On the other hand, Polyurea can be made in high-moisture conditions without a catalyst which is a huge advantage.
However, neither Polyurea nor polyurethane can be applied to a wet substrate. The ambient temperature and substrate temperature may have little effect on the reaction and cure of a polyurea system compared to the other. This was noticed in one study where samples of steel, concrete, and asphalt coated with a polyurea elastomer in the presence of a stream of liquid nitrogen (-196degC) sprayed for 30 seconds showed no damage.
Polyurea coatings have a very short initial cure time and, therefore, may be used in an aggressive environment, where high curing speed is required (e.g., an oil pipeline). Some polyureas are fast air-drying (less than 30 minutes, even less than 10 seconds for some cases for pure Polyurea).
There are many options for polyurethanes, ranging from a few minutes to several hours.
Polyurethane and Polyurethane Coatings Performance Characteristics
All polyureas (aromatic) systems will experience gloss loss, chalking, and discoloration from ultraviolet (UV), but this does not impact their quality or performance. UV stabilizers and antioxidants will not prevent coating discoloration and fading over time when used in light colors for exterior exposure.
As for polyurethanes, the addition of pigments will improve the resistance to UV exposure. The aliphatic polyurethanes will not deteriorate in UV light, while the aromatic polyurethanes can withstand UV damage and are used for exterior coating formulations. However, they will quickly chalk in UV light.
Polyurethane and polyurethane systems have similar resistance to chemical products because they share the same polyether backbones in the elastomer. Polyurea systems with new technologies have different polymer backbones. These backbones can be epoxy, silicone, or a combination thereof. The polyurea systems are generally resistant to gasoline, acetone, and tertiary methyl ether (MTBE), as well as sulfuric acid at low concentrations (5%). See Table 1
The chemical resistances for polyurethane are listed below in Table 2 and should always be checked with manufacturing data.
Useful Tips for Polyurea and Polyurethane Coatings
The Society for Protective Coatings (SSPC) guides methods for applying polyurethane and polyurea coatings:
- SSPC-Paint39 Two-Component Aliphatic, Polyurea Topcoat
- SSPC-PS 28.01 - Two-Zinc Rich Polyurethane Primer/Aliphatic Polyurea topcoat System
- SSPC-Paint43 Direct-to-Metal(DTM) Aliphatic PolyureaCoating
- SSPC-PA14 Application of Thick-Film Polyurea and Polyurethane Paints to Concrete and Steel Using Plural Component Equipment
- SSPC-Paint 45 Two-Component Thick-Film Polyurea and Polyurea/Polyurethane Hybrid Coatings
Polyurethanes can be applied using standard spray, brush, and rollers.
Polyurea and Polyurethane Coatings: Uses
An epoxy primer is required for many polyureas as their adhesion may not be sufficient to withstand heavy-duty service. Because of their short cure time, they are often used with special equipment such as plural component spraying using a spray gun with a mixing head. However, all guidance that is provided by the manufacturer regarding the mixing of multi-component products, thinning requirements, induction times, and special application requirements should be followed.
These aromatic polyurethane floors are suitable for many different levels of chemical and mechanical exposure. They can also be used on concrete, plywood, metal, and wood surfaces. A waterproof membrane can be used in certain cases to extend the application. This coating type can be applied in thick film for immersion service; some formulations allow up to 500 microns (20 mils) or more per coat for industrial and marine applications.
Aliphatic polyurethane coatings (high-performance two-component UV-stable topcoats) are typically used for exterior coatings. However, some formulations are suitable for exposure in environmental zones 1A (interior, normally dry), including offshore structures, chemical and petrochemical plants, bridges, pulp and paper mills, and the power industry, excluding immersion zones.
Polyurea coatings provide high performance in industrial applications requiring high abrasion, chemical, and corrosion resistance. They can be used on concrete, metal, wood, ceramic, and other materials for waterproofing and protection. (To learn more about polyurea coatings, read The History and Industry Adoption of Polyurea Coating Systems.)
Different raw materials in chemical reactions and the use of catalysts to accelerate the reaction time can cause marked differences in the sensitivity and environmental conditions that affect the application and cure times of the products. Other physical and chemical characteristics can also be used to indicate different properties.
Polyurea and Polyurethane coatings serve as excellent options for many applications. However, the curing time characteristics and the versatility of the application process under extreme environmental conditions makes polyurea coatings the most cost-effective system despite its raw material cost, which is more expensive than most other systems, including polyurethane.