Polyurea Spray-Applied Systems for Concrete Protection


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Polyurea spray coatings technology is a recent development in the polyurethane coatings industry. Polyurethane chemistry has been in existence for approximately 60 years, while elastomeric urethane coatings have been accessible since the 1970s. Polyurea elastomer technology was introduced a decade later. The two main administration 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 øC), and insensitivity to humidity, with exceptional physical properties such as hardness, flexibility, tear, and tensile strength, and chemical and water resistance. This results in excellent weathering and abrasion resistance. The systems are complete solids, making them compliant with the most stringent VOC regulations. Due to its specific curing profile and exceptional film properties, the polyurea spray coating technique has been introduced into many areas, including corrosion protection, containment, membranes, linings, and caulks.

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Figure 1: Isocyanate Chemistry
The term 'polyurea' has been improperly used in the past. Urethane coatings chemistry can be split into three segments: polyurethane coatings; polyurea coatings; and hybrid polyurethane/polyurea coatings. All three are linked to different isocyanate reactions (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 additives can be introduced to all of them.
A 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 that is a combination of the above-mentioned dual-coating systems. The isocyanate component can be the same as the "pure" polyurea systems.

The resin blend is a blend of amine-terminated and hydroxyl-terminated polymer resins 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 resins, the addition of one or more catalysts is necessary.
The water/isocyanate reaction also provides 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 dominated by the much slower isocyanate-water reaction and produces carbon dioxide.



Figure 2 / Applicability of the Different PU Chemistries
The Polyurethane Landscape
The choice between the different polyurethane (PU) technologies is based on 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 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. Moreover, because the catalyzed 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 shallow 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;
an application under high humidity and at low temperatures;
extreme abrasion resistance;
impermeable membranes;
high thickness build up;
chemical resistance.



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.
The market development started in the United States, followed by Asia, with robust growth during the second half of the 1990s. In the first stage of development, 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.

The large window of application conditions, with a high tolerance for humidity, both from the environment and from the substrate and temperature, makes polyurea a very suitable coating for concrete in construction applications such as roof repair, containment liners, membranes, car park decks, bridges and offshore. The high abrasion resistance leads to its use in coatings for truck, bulk transport wagons, freighters, and conveyor belts. Table 2 presents an overview of the application fields where polyurea is chosen based on one or more of its unique application and film properties.



Raw Materials
A polyurea spray coating formulation consists of five different elements: the isocyanate component; (reactive) diluent; polyetheramines; chain extenders; and additives, fillers, and pigments.


Isocyanate
Since the most commonly used isocyanate is diphenylmethane diisocyanate (MDI), this paper 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 to 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 provides an effective mixture of the two components. However, they become much more reactive, with the risk of building up more internal stress. Higher-NCO prepolymers will be used if higher surface hardness is needed. Table 3 provides an overview of the main properties of the MDI prepolymers used for polyurea spray coatings in Europe.


Chain Extenders
Diethyl-toluene diamine, 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 like dimethylation-toluene diamine (DMTDA), N,N'-di(sec.butyl)-amino-biphenyl methane (DBMDA) or 4,4'-methylene-bis-(3-chloro, 2,6-diethyl)-aniline (MCDEA) slow down the reaction significantly. Table 5 lists various chain extenders and their characteristics. Significantly slowing down the reaction also means that the competition with the water reaction becomes more important, and precautions need to be taken.


Additives, Fillers, Pigments
Depending on the application, solvents, additives, pigments, and fillers are introduced to the formulation. Adhesion promoters like silanes are used to enhance the adhesion on steel and concrete. UV absorbers are used to slow down the yellowing effect of aromatic polyurea systems. Fillers are added to lower the raw material cost and improve the physical properties of the coating. The addition of pigment and 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 crucial element of handling polyurea coatings is mixing. Proper mixing will be obtained in a suitable mixing module by impingement with the mechanical purge. The operational pressure and temperature of the products will also help to optimize the mixing efficiency.
Due to the high cure speed of polyurea and the short mixing time, the products are mixed by impingement at high pressure. Indeed, for field applications, it is preferable to formulate the products on a fixed 1:1 volume-mixing ratio. The weight 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 thickness of the two components needs to be at the same level. The properties of these prepolymers can be found in Table 3. The viscosity of the resin blend at 25 deg C is approximately 900 mPa.s, dropping below 100 MPa. Sat application temperature.

Experiments prove that polyurea films produced at 65 deg C, 70 deg C, and 80 deg C have different properties, and these properties improve with increasing temperatures. The spraying equipment has improved significantly. The new spray equipment allows different temperature settings for the two components, ensuring an optimum mixing in the spray head. Other features are more comfortable variable ratio settings; easy output control; and 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 application. The film properties of the 1:1 volume ratio sprayed system was 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 small index shifts.


Aspects of Spray Polyurea Technology
Polyurea application had some problems during the initial start-up phase, which are at the origin of the still-existing misconceptions about polyurea technology. These problems can be attributed partly to the lack of experience at the time of the technology introduction, partly due 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 current coatings systems.
Initially, polyurea spray coatings looked too easy to apply. Polyurea is very fast - the surface can be put into service immediately after the application, and the final properties of the coating are obtained only a few hours afterward. Polyurea is not water- or temperature-sensitive and is easy to formulate and produce. The first systems on the market were indeed high-speed, with a gel time of fewer than two seconds, and initially, several problems were linked to the reactivity of the systems.

At first, substrate wetting was a problem. This problem was linked to the development phase of polyurea with the use of high-speed spray systems. Development programs focusing on adhesion on concrete, with polyurea systems presenting gel times of three to four seconds, resulted in cohesive adhesion failure in the concrete. In practice, to limit the risks under variable field conditions, a multi-layer system is applied, made of a primer and a topcoat.

A second problem noted in the field was the lack of intercoat adhesion. Lab tests, with times between coats of several weeks, have shown that intercoat adhesion is excellent. When problems occur with intercoat adherence, most of the time, they can be related to issues 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 towards the mixing module, can cause poor mixing. Adapting the machine settings of the spray can solve this.

Due to the high reactivity of the systems, the surface quality of the sprayed film was initially inferior. Fine-tuning the spraying equipment was a first step towards solving the 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.

The cost of polyurea spray coatings technology is seen as a barrier to entry. "Pure" polyurea systems are more expensive, when considering raw materials cost alone, but can be applied in areas where all other orders will fail or where they are not suitable. Also, the initial investment in equipment is rather costly.

However, when estimating the capital cost for a project, polyurea is more competitive when both the processing time and the waiting period before the coated substrate is put back into service are included.

As discussed 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.


Construction-Related Aspects
Polyurea spray coatings can be used under severe weather conditions because the chemistry is very fast, and they do not experience adverse side effects caused by the presence of humidity. They also cure at temperatures below 0 deg C. However, when using polyurea coatings, several precautions still have to be taken.
If primers are considered, a right approach is to start with the evaluation of existing primers with known performance on the substrate. It is essential to determine the adhesion performance of the polyurea coating on the textbooks and to check whether the application conditions of the introduction and the re-coating conditions for the introduction still work for a system with polyurea as a finish.


Concrete Surface Defects and Surface Preparation
The low cost, high strength, and structural properties of concrete make it the material of choice for the construction industry. Some of the typical features of concrete like the limited chemical resistance, dust release, and porosity or permeability make it necessary to put a protective and decorative layer onto the surface.
The preparation of the surface is essential. Depending on the surface quality of the concrete, one or more of the following actions need to be taken:

the water jet and solvent cleaning;
grit blasting;
bunghole and crack filling;
repair layer of concrete;
priming.


Steel Surface Defects and Surface Preparation
The life cycle of steel construction coatings depends mainly upon the protection system put in place. The life of the protective coating itself is strongly dependent on the surface condition before the application of the sheet. The protection of the substrate is mainly obtained by ensuring excellent adhesion. Two adhesion mechanisms are possible:
the molecular attraction of the interfacial forces from both the coating and the substrate,
mechanical bonding or anchoring of the layer on the substrate.
Depending on the condition of the surface, one or more of the following pre-treatments will be necessary: cleaning and degreasing with solvents, water jet or detergents; hand or power-tool cleaning; grit blasting.
On freshly grit-blasted, dust-free steel with a surface roughness SA 2 1/2 to SA 2 according to the specification ISO 8501-1, very high adhesion values can be obtained for polyurea, even without the use of primers.



Figure 4 / Mixing Efficiency Influence on the Physical Properties of a Polyurea Coating
Mixing Efficiency
The mixing efficiency of the application equipment is of vital importance. When formulating a system or modifying an existing system, it is necessary to verify the mixing efficiency always. Figure 4 shows the influence of changing the mixing on the physical properties of a formulated product. The tensile strength almost doubles from 14 N/mm2 to 23 N/mm2, the angle tear increases from 75 N/mm to 85 N/mm and the elongation increases from 390 to 430 percent. In this case, the influence on the other physical properties is limited.


Figure 5 / Physical Property Variations at Varying System Index
System Index Influence on Final Film Properties
Earlier experiments reveal that a polyurea coating needs to be formulated at an index above 1.00, meaning with a slightly higher amount of isocyanate-groups than amine-groups. At an index of 1.00 or lower, the physical properties of the coating become unreliable. Figure 5 demonstrates that most properties have excellent values at indexes from 1.10 to 1.30. Above an index of 1.30, the performance tends to drop again.
Taking into account that, in practice, slight variations might occur in the application parameters, related to the precision of the spray equipment and changes in application conditions, it is safer to work at a minimum index of 1.10 to 1.15.



Figure 6 / Physical Property Development as a Function of Filler Content
Filler Influence on Physical Properties
Adding fillers to a polyurea system can be useful for different reasons, such as a reduction in the raw material cost or improvement in physical properties. Inorganic fillers have different hardness, and some will abrade parts of the spray equipment more than others. The most sensitive parts are the mixing chamber and the nozzle of the spray gun. The filled systems need to be carefully filtered before packaging.
Depending on the performance of the spray installation, the dosing of filler can vary. We added up to 40% of padding to the resin blend. The main limiting factor for the processing is the increase in viscosity of the filler component. This can result in difficulties with the pumping unit and poor mixing due to significant differences in thickness for both parts.

Adding fillers improve the surface hardness, the angle tear, and flexural bending modulus but has a negative influence on the elongation and the tear propagation or trouser tear (Figure 6).



Figure 7 / Water Absorption for Different SUPRASEC Prepolymers
Water Absorption
For corrosion protection, the primary consideration for excellent performance is adhesion to the substrate. Further testing proved that, even with excellent adhesion, the resistance to cathodic disbondment for some systems fails. Since cathodic disbondment occurs over a longer term and certainly is not a simple test method, we measured the water absorption over a period of 10 days at 80 deg C and added 3% sodium chloride to the water.
As can be seen in Figure 7, the unmodified standard polyurea spray coatings give only limited protection against corrosion. The water absorption drops significantly with increasing the NCO-content for the prepolymer. Further fine-tuning of the prepolymer resulted in water absorption is well below 0.5% after ten days. Cathodic disbondment testing on this system gave very satisfying test results.


Antiskid Performance
Flooring, car parks, and sports floors are critical applications for polyurea due to its high abrasion resistance, excellent mechanical properties, and insensitivity to blistering during the curing process in humid conditions.
One example is the outdoor, impact-absorbing playground flooring. The system tested is based on SUPRASEC 2049, formulated to be applied in a 1 to 1 volume ratio. The film properties are 70 Shore A hardness, 600% elongation, and 13 N/mm2 tensile strength. The results in Table 6 show that it is entirely possible to formulate a system, which complies with the antiskid needs of a flooring system, in this case, for a flexible substrate.


Conclusions
Polyurea spray coating technology is different from other coating chemistries and can expand the application range of coatings to areas and conditions where other coating systems will fail.
Polyureas are very suitable for construction applications. The fast curing makes it possible to use when only short disturbance periods are allowed. The fact that the isocyanate/water reaction is not affecting the physical properties of the applied film expands the use of polyurea to high relative humidity conditions and does not set such stringent limits on the water content of substrates like concrete. Although they slow down at colder temperatures, polyurea coatings still cure at temperatures where other chemistries fail.

The formulation of polyurea spray coatings has to be approached similarly to any other coating system. Careful selection of raw materials for fine-tuning of the formulation and evaluation of the policy, in the sometimes difficult conditions where the coating is to be applied, is still necessary.

Polyurea spray coating technology means handling reactive chemicals. While managing the chemicals during their manufacturing, packaging, and application, the correct protective clothing should be worn at all times.

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