THE PHYSICAL PROPERTIES OF POLYURETHANE & POLYUREAS

The polyurea and polyurethane systems have similar properties in general when compared to different generic coatings and system like epoxies, polyester/FRP systems and other rubbers and elastomers. Both polyureas and polyurethanes have high tensile strength, flexibility, impact resistance, abrasion resistance and can be formulated into a wide variety of property ranges. However, when one looks more closely at the polyurea and polyurethane physical properties, each has unique properties which allows one of the two systems to be more viable or optimum in an application than the other.

Elongation/Modulus

For example, polyurethanes can be formulated with higher elongation and in a lower modulus than a polyurea with a similar hardness. The modulus is the amount of force required to stretch an elastomer to its ultimate elongation. Polyurethanes typically have lower modulus than polyureas, meaning they feel softer, more elastic and will stretch with less force which could be important for specific applications. Applications like waterproofing membranes and some abrasion resistant applications require a low modulus and a high elongation elastomer.

Application Temperatures

Both polyurea and polyurethane cure very rapidly, even at ambient temperatures well below room temperature. However, the auto catalytic nature of the polyurea allows them to be applied at 20-30oF (-6.7 to -1.1oC) lower than polyurethane systems. This is an important factor when applying systems to substrates like cold steel which typically is a heat sink substrate. Many times, applying conventional urethane systems to heat sink substrates could comprise the properties or adhesion of the first layer or pass, whereas there would be no problem using a polyurea system. Typically, Futura designs a polyurea/polyurethane blend for applications over heat sink surfaces.

Abrasion Resistance

Polyurea and polyurethane have excellent abrasion resistance in general. However, in critical applications where abrasion resistance is a key requirement, polyurethane elastomers will provide 2-3 times better abrasion resistance than a polyurea system.

Heat Resistance Polyureas have the advantage of having higher heat resistance than a comparable formulated polyurethane. Polyurea can be formulated in a structural plastic that has flexibility and high impact/toughness yet still has excellent heat distortion resistance/heat sag resistance. A structural urethane plastic can be formulated to have similar impact resistance and flexibility but would not have as good heat distortion resistance or heat sag properties.

Adhesion

There are claims that polyurea can be applied directly over steel, roofing substrates and other substrates without any primer and minimal surface preparation. If something sounds too good to be true, it probably is. And adhesion problems may occur later on. The polyureas do not have any additional adhesion characteristics than a polyurethane elastomer. They may appear to adhere well to sandblasted steel or directly to various substrates. While the adhesion may be adequate in non-critical applications, a primer is necessary for immersion applications or dynamic applications like abrasion resistant applications

Weatherability

Polyurethanes and polyureas are available in systems based on aliphatic isocyanate prepolymers which are higly weatherable and color retentive, and systems based on aromatic isocyanate prepolymers which are not color stable and will tend to chalk and darken in color. Presently, the aromatic polyurea is the main version of Polyurea that is promoted in the market. Futura's side-by-side testing has identified that most aromatic polyurea systems will discolor and chalk sightly more than polyurethanes. The polyetheramine and other amine reactants in a typical polyurea formula have a tendency to yellow somewhat more than the hydroxyl polyol based polyurethane.

Chemical Resistance

In general, the chemical resistance of the polyurea and polyurethanes are very similar when exposed to diluted acids, alkali and salt solutions. Where they differ is in the organic solvents and oils. The polyurea systems tend to sell significantly more than the polyurethanes and there may need to be some caution in the environment.