Polyurea spray coating technology is a recent development in the polyurethane coatings industry. 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 deg C) and insensitivity to humidity to exceptional physical properties such as high hardness, flexibility, tear and tensile strength, and chemical and water resistance. The result is good weathering and abrasion resistance. The systems are 100%-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

Figure 1 / Isocyanate                    Reactions                 Click here to enlarge

The term 'polyurea' has been used incorrectly in the past. Urethane coatings chemistry can be divided into three subsegments: polyurethane coatings, polyurea coatings and hybrid polyurethane/polyurea coatings, all linked to different isocyanate reactions (see Figure 1). Each of these segments deals with systems that 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.

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.

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 hydroxyl-terminated resins can be used. Resin blends should only contain amine-terminated resins and/or chain extenders, and no hydroxyl-reactive polymer components. All the polyurea coatings mentioned in this article comply with this requirement.

A polyurethane/polyurea hybrid coating is composed of a combination of these two coating systems. The isocyanate component can be the same as for the "pure" polyurea systems. The resin blend is a blend of amine-terminated and hydroxyl-terminated polymer resins and/or 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 as the amine-terminated resins, 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 a polyurea reaction since the mechanism is a two-step process, which is controlled by the much slower isocyanate/water reaction, and produces carbon dioxide.
 

The Polyurethane Landscape

The choice between the different polyurethane (PU) technologies is based upon different parameters (see Figure 2). Polyurethane presents the best compromise between cost and quality, but is limited by 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 catalyzed polyol/isocyanate reaction behaves differently to changing application temperatures than the amine/isocyanate reaction, 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 -20 deg 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/or at low temperatures
• Extreme abrasion resistance
• Impermeable membranes
• High thickness buildup
• Chemical resistance

The Applications for Polyurea Coatings

To define the right applications, a good understanding of polyurea spray coatings properties is needed. 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 very good abrasion resistance.

The market development started in the United States, followed by Asia, with a very strong growth during the second half of the 1990s. In its first stage, polyurea was used as a protective layer over polyurethane insulation foam for roofing applications. In Europe, the polyurea spray coatings market only started to develop in the last few years.

Due to its high tolerance for humidity, both from the environment and from the substrate and temperature, polyurea is suitable for coating concrete in construction applications such as roof repair, containment liners, membranes, car park decks, bridges and offshore applications. Its high abrasion resistance allows application in liners for truck, bulk transport wagons, freight ships and conveyor belts.
 

Raw materials

A polyurea spray coating formulation consists of the following five components.

• Isocyanate
• (Reactive) diluent
• Polyetheramines
• Chain extenders
• Additives and pigments

Isocyanate

Since the most commonly used isocyanate is diphenylmethane diisocyanate (MDI), this article focuses on MDI-based products. Aliphatic systems can be used where UV stability is an issue.

Standard polyurea spray coatings use MDI-prepolymers with an NCO content of 15-16%. In this NCO range, a good compromise between viscosity of the material and the reactivity of the system is obtained. Lower NCO prepolymers have a higher viscosity, but give higher elasticity and slower reactivity. Higher NCO prepolymers are lower in viscosity, which is good for an effective mix of the two components. However, they become much more reactive, with the risk of building up more internal stress. Higher NCO prepolymers may be used if higher surface hardness is needed. Table 2 provides an overview of the main properties of the MDI-prepolymers used for polyurea spray coatings in Europe.
 

Diluent

Propylene carbonate is a reactive diluent for polyurea; it has a high flash point, low toxicity and should not be considered a VOC.

Following are the main advantages of using propylene carbonate.

• Improved shelf life of the isocyanate-prepolymer;
• A compatibilizer for the mixing of the two components in the mixing chamber of the spray gun;
• A viscosity reducer for isocyanate-prepolymers;
• Improved leveling of the applied film.

Propylene carbonate reacts with an amine to give a carbamate structure containing a secondary hydroxyl group (see Figure 3). Because of the quick reaction between isocyanate and amine, the secondary hydroxyl does not get a chance to react with an isocyanate group. The propylene carbonate molecule should, therefore, be considered a mono-functional molecule.

In applications where contact with water cannot be avoided, the use of propylene carbonate should be limited since propylene carbonate is completely miscible with water and unreacted propylene carbonate could be extracted, increasing the water permeability of the film.

Other solvents or viscosity reducers can be used if they are compatible with the isocyanate component. They may be considered a VOC. However, they will increase the shrinkage effect.
 

The amine blend used in polyurea spray coatings is a mixture of polyetheramines and chain extenders. The main component of the resin blend is a mixture of amine terminated ethylene oxide and/or propylene oxide polyether with molecular weights varying from 200 to 5000 g/mole. The primary amine groups provide a very fast and reliable reaction with the NCO groups of the isocyanate component. Table 3 represents the properties of the polyetheramines commonly used in polyurea.

 

Chain Extenders

Diethyltoluenediamine, or DETDA, is the standard chain extender used in aromatic polyurea spray coatings. DETDA contributes to the hard block and improves the heat resistance of the cured film. It is the most reactive amine in the resin blend but, because of the phase separation during the curing, it controls the reaction mechanism and makes it possible to spray a polyurea film.

Other chain extenders, such as dimethylthio-toluenediamine (DMTDA), N,N'-di(sec.butyl)-amino-biphenyl methane (DBMDA) or 4,4'-methylenebis-(3-chloro, 2,6-diethyl)-aniline (MCDEA) slow down the reaction significantly. Table 4 lists various chain extenders and their characteristics.

Slowing down the reaction also means that the competition with the water reaction becomes more important and precautions need to be taken.
 

Additives and Pigments

Depending on the application, additives, pigments and/or fillers are introduced to the formulation. The addition of pigment and/or fillers is limited because the viscosity of the two components at the application temperature has to be kept under control. Higher amounts of fillers and reinforcement fillers can be added to the system as a third component.

Product Application Specifics

The most important element of handling polyurea coatings is the mixing. Good mixing will be obtained in a suitable mixing module by impingement with mechanical purge. Operational pressure and temperature of the products will also help to optimize the mixing efficiency.

Because of the high cure speed of polyurea and the short mixing time, the products are mixed by impingement at high pressure. Certainly for field applications, it is preferable to formulate the products on a fixed 1:1 volume-mixing ratio. The pressure used in the field will vary between 150 and 250 bar. The viscosity of the products at application temperature ideally needs to be lower than 100 mPa.s and the viscosity of the two components needs to be at the same level. Figure 4 shows the viscosity of the MDI-prepolymers at different temperatures. The properties of these prepolymers can be found in Table 2.

The viscosity of the resin blend at 25 deg C is approximately 900 mPa.s, dropping below 100 mPa.s at application temperature.

Experiments have proven that polyurea films produced at 65 deg C, 70 deg C and 80 deg C have different properties and that these improve with increasing temperature. The new spray equipment allows different temperature settings for the two components, ensuring an optimum mixing in the spray head.

The spraying equipment has improved significantly. New features include the following.

• Separate temperature settings for both components
• Easier variable ratio settings
• Easy output control
• Easy monitoring of application parameters

The index of a polyurea system is typically kept at a slight over-index of the isocyanate in the range of 1.05-1.10. As the isocyanate-group reacts to humidity, the excess isocyanate compensates for the 'loss' of isocyanate-groups during storage and/or application. The film properties for 1:1 volume ratio sprayed system were measured for an index variation between 0.90 and 1.15. The test results indicate that the film performs best at an index of 1.05 and higher. Below an index of 1.05 the results can vary significantly and become unpredictable, even for an index shift of 0.2.

Misconceptions/Past Problems

The application of polyurea has known some problems during the initial start up phase, which are at the origin of the misconception still existing about polyurea technology. These problems can be attributed partly to the lack of experience at the time of the introduction of this technology, partly to the lack of adequate application equipment, and partly to the fact that this new technology could not be applied in the same way as the existing coating systems.

When polyurea was first presented, the initial misunderstanding started because it simply looked almost too easy to apply. Polyurea is very fast, the coating can be put in service immediately after the application and the final properties of the coating are obtained only a few hours afterward; it is not water or temperature sensitive, and is easy to formulate and produce. Following is a list of common concerns resulting from unsuccessful trials and their solutions.

The system is too fast; intercoat adhesion is therefore bad.

The first systems on the market were indeed very fast, with a gel time of maximum 2 seconds. Lab tests with times between coats of several weeks have shown that intercoat adhesion is very good, also with these very fast systems. When problems occur with intercoat adhesion, most of the time they can be related back to problems with the raw materials, the manufacturing of the systems or the spray equipment. Spray equipment problems or a disturbance of the feeding of one or both components toward the mixing module can cause poor mixing. Adapting the machine settings of the spray can solve this.

Surface quality is very poor.

Due to the high reactivity of the systems, the surface quality of the sprayed film was initially very poor. The fine-tuning of the spraying equipment was a first improvement for solving that problem. The use of non-VOC, reactive diluents and the development of new MDI prepolymers with higher 2,4'-isomer content resulted in perfect surface quality without compromising on working time.

Too fast buildup of layer thickness.

The initial applications of polyurea spray coatings resulted in high layer thickness. Further development of the spray equipment provided access to applications where only a few hundred microns were needed. Industrial applications looking for 100-150 microns will be available in the near future.

Substrate wetting is very poor.

Again, this was a problem linked to the development phase of polyurea with the use of extremely fast spray systems. Development programs focusing on adhesion on concrete, with polyurea systems presenting gel times of 3-4 seconds, resulted in cohesive adhesion failure in the concrete. In practice, to limit the risks under variable field conditions, a multilayer system is applied, made of a primer and a topcoat.

Cost of the system (raw material cost vs. project cost and product life cycle).

"Pure" polyurea systems are more expensive, when considering the raw materials cost alone, but can be applied in areas where any other systems will fail. While estimating the capital for a project, polyurea is even more competitive, when both the processing time and the waiting period are included, before putting the coated substrate back in service.

Equipment investment

The initial investment in equipment is rather costly. As highlighted above, the success of the project is very equipment- and applicator-dependent, and we believe that the high entry barrier can only guarantee quality services from specialized and skilled operators.

Experienced applicators.

The experience and motivation of well-trained applicator teams is as important as the spray equipment, the choice of raw materials, the adequate personal protective equipment, and the formulation of a suitable system.

Latest Developments

Influence of the Functionality of the System (Table 5)

Polyurea coatings are known for their chemical resistance and toughness. For some application areas, such as primary containment and inside oil-pipe coatings, high chemical resistance is needed. A way to improve chemical resistance is to increase crosslink density. The following conclusions can be made from one of our experimental programs.

With increasing functionality of the polyurea system:

• The viscosity of prepolymer increases and the mixing becomes less efficient.

 

• The gel and tack-free time decreases and the surface quality becomes very poor. At prepolymer functionalities above 2.5 the systems cannot be processed anymore.

 

• The physical properties improve until a functionality of about 2.5 is reached; above 2.5, the physical properties decrease with the increasing functionality of the polyurea system.

 

• The flexibility of the system decreases.

 

• The abrasion resistance decreases.

 

• The chemical resistance decreases.

The expected improvements of physical and chemical resistance properties are not obtained because of the loss of crosslink efficiency with increasing functionality of the polyurea system. In a typical formulation with a gel time of 2-4 seconds, a functionality of 2.2 or below should be taken.

2,4'-Isomer Content of the Prepolymer

The higher sterical hindrance of the 2,4'-isomer has an effect on the reaction time. The longer gel time and open time result in the following.

• Less critical application conditions setting
• Better substrate wetting
• Superior adhesion on a variety of substrates
• Improved leveling behavior resulting in a better surface quality
• Enhanced physical properties (see Table 6)

Fast Property Development

Polyurea coatings are mainly used in applications where fast curing is needed. When used as a corrosion protection on pipes, storage of high volumes of coated pipes is difficult and, because of the weight, the coating needs to be fully cured before being transported. Polyurea used for pier protection needs to be fully cured before high tide.

A polyurea coating builds up high internal stress during the initial phase of the curing due to the high reactivity. The stress has a negative influence on the development of the physical properties of the freshly applied polyurea film. By modifying the MDI prepolymer, it is possible to improve the relaxation of the coating and thereby reduce the waiting period before the applied coating can be put in service. The improved relaxation results in less stress buildup, no deformation, improved adhesion and faster physical properties development.
 

For this study, the cold impact resistance was tested, along with standard physical properties (see Table 7a). Cold impact testing, within the hour after application, proved to be a very reliable measurement of property development as a function of time. The goal was to develop the properties faster without compromising the end properties (see Table 7b).