TEKPOL | Polyurea

Polyurea

Polyurea spray coatings technology is a recent development in the polyurethane coatings industry. The polyurethane chemistry is about 60 years old. Since the 1970s elastomeric urethane coatings have been available. The polyurea elastomer technology was introduced some 10 years later. Two main application areas are Reaction Injection Moulding (RIM) and sprayable coatings. Polyurea coatings combine extreme application properties such as rapid cure, even at temperatures well below 0°C, and insensitivity to humidity, to exceptional physical properties such as high hardness, flexibility, tear strength, tensile strength, chemical and water resistance. The result is good weathering and abrasion resistance. The systems are 100 percent-solids, making them compliant with the strictest VOC regulations. Due to its specific curing profile and exceptional film properties, the polyurea spray coating technique developed into various areas, including corrosion protection, containment, membranes, linings and caulks.

Definition

The term 'polyurea' has been wrongly used in the past. The urethane coatings chemistry can be divided into three sub segments: i/ polyurethane coatings, ii/ polyurea coatings, and iii/ 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/or additives can be introduced to all of them.

1)A purely 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 contain one or more catalysts.

2) 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/or hydroxylterminated resins can be used. On the other hand, the resin blend should only contain amine-terminated resins and/or chain extenders and not any hydroxyl reactive polymer components. All the polyurea coatings mentioned in the paper comply with this requirement.

The choice between the different polyurethane (PU) technologies is based upon different 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 more than 5% humidity. This is due to the competition between hydroxyl-polyols and water for the reaction with an isocyanate group. Humidity content of the environment and the application temperature are limiting factors for polyurethanes and other chemically reacting systems. Hybrid systems already have a larger scope of application conditions, but the presence of catalysts in hybrids makes them more sensitive to humidity than "pure" polyurea systems. Moreover, because the catalysed polyol/isocyanate reaction behaves differently from the amine/isocyanate reaction to changing application temperatures, the system 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 very low temperatures (as low as minus 20°C) the polyurea coating will still cure. Polyurea coatings combine high flexibility with hardness.