What Is Polyurea?

The term ‘polyurea’ has been wrongly used in the past.
The urethane coatings chemistry can be divided into three sub-segments: polyurethane coatings, polyurea coatings, and hybrid polyurethane, polyurea coatings, all linked to different
isocyanate reactions.

(Figure 1). Each of these segments deals with systems, which can be aromatic, aliphatic, or a blend of both aromatic and aliphatic.
Pigments, fillers, solvents, and additives can be introduced to all of them.
Figure 1: Isocyanate chemistry, clear polyurethane coating is the result of a reaction between an isocyanate component and a resin blend made with only hydroxyl-containing resins.
The final coating film will contain no intentional urea groups. A polyurethane system will most probably include one or more catalysts. A polyurea coating is the result of a one-step reaction between an isocyanate component and a resin blend component. The isocyanate can be monomer based, a prepolymer, a polymer or a blend. For the prepolymer, amine- and hydroxyl terminated resins can be used. On the other hand, the resin blend should only contain amine-terminated resins and chain extenders and not any hydroxyl reactive polymer components.

All the polyurea coatings mentioned in the paper comply with this requirement. A polyurethane/polyurea hybrid coating has a composition which is a combination of the two layers mentioned above systems. The isocyanate component can be the same as the “pure” polyurea spray systems. The resin blend is a blend of amine-terminated, hydroxyl-terminated polymer resin and chain extenders. The resin blend may
also contain additives or non-primary components.

To bring the reactivity of the hydroxyl-containing resins to the same level of responsiveness as the amine-terminated
adhesives, the addition of one or more catalysts is necessary.
The water/isocyanate reaction also produces urea-groups at the end of the process.
However, this reaction should not be considered to be a polyurea reaction as the
mechanism is a two-step process, which is controlled by the much slower isocyanate-water response, and produces carbon dioxide.

2- The polyurethane landscape
The choice between the different polyurethane (PU) technologies is based on various parameters (Figure 2). Polyurethane presents the best compromise between
cost and quality but is limited by the application performance. The polyurethane system
is susceptible to blistering when the substrate contains over five percent humidity. This is due to competition between hydroxyl-polyols and water for the reaction
with an isocyanate group. The humidity content of the environment and the application temperature are limiting factors for polyurethanes and other chemically reacting
systems.

Hybrid systems already have a broader scope for application conditions, but the presence
of catalysts in hybrids makes them more sensitive to humidity than “pure” polyurea
Systems. Because the catalyzed polyol/isocyanate reaction behaves
differently from the amine/isocyanate reaction to changing application temperatures, the
the order becomes less robust.

Polyurea can be used in extreme conditions. When it is used on substrates, almost
saturated with water, polyurea will not provoke blistering nor will blistering occur when the air contains high amounts of humidity.
Even at shallow temperatures (as low as -20°C), the polyurea coating will still cure. Polyurea coatings combine high flexibility with hardness. They are the most suitable coatings when the following is required:

· high curing speed,
· application under high humidity and at low temperatures,
· extreme abrasion resistance,
· impermeable membranes,
· high thickness build up,
· chemical resistance.

Figure 2: Applicability of the different PU chemistries
3- The applications for polyurea coatings
A good understanding of the properties of polyurea spray coatings is required to specify the right form. Table 1 provides a general overview of the physical and chemical
properties that can be expected of polyurea spray products. Polyurea systems are
known to be very tough. They combine high elasticity with high surface hardness,
resulting in excellent abrasion resistance.