Polyurea coatings combine severe application homes such as quick cure (even at temperature levels well below 0 øC), and insensitivity to humidity, with remarkable physical residential or commercial properties such as high hardness, flexibility, tear and tensile strength, and chemical and water resistance. This results in great weathering and abrasion resistance. The systems are 100% solids, making them certified with the strictest VOC regulations. Due to its specific treating profile and exceptional movie residential or commercial properties, the polyurea spray coating method has been introduced into numerous locations, including corrosion protection, containment, membranes, linings and caulks.
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Definitions
A polyurethane coating is the outcome of a reaction between an isocyanate element and a resin mix made with only hydroxyl-containing resins. The final coating movie will include no deliberate urea groups. A polyurethane system will most likely include one or more catalysts.
A polyurea coating is the outcome of a one-step reaction in between an isocyanate part and a resin-blend element. The isocyanate can be monomer based, a prepolymer, a polymer or a mix. For the prepolymer, amine- and/or hydroxyl-terminated resins can be used. On the other hand, the resin mix must only include amine-terminated resins and/or chain extenders and not any hydroxyl reactive polymer parts. All the polyurea coatings mentioned in the paper adhere to this requirement.
A polyurethane/polyurea hybrid coating has a structure that is a combination of the above-mentioned two-coating systems. The isocyanate component can be the like the “pure” polyurea systems. The resin blend is a mix of amine-terminated and hydroxyl-terminated polymer resins and/or chain extenders. The resin blend might also consist of additives, or non-primary parts. To bring the reactivity of the hydroxyl-containing resins to the same level of reactivity as the amine-terminated resins, the addition of one or more drivers is necessary.
The water/isocyanate response likewise produces urea-groups at the end of the procedure. However, this reaction should not be thought about to be a polyurea response as the mechanism is a two-step procedure, which is managed by the much slower isocyanate water response and produces carbon dioxide.
The Polyurethane Landscape
The option in between the various polyurethane (PU) innovations is based upon various specifications (Figure 2). Polyurethane presents the very best compromise between cost and quality, but is limited by the application performance. The polyurethane system is susceptible to blistering when the substrate consists of more than 5% humidity. This is because of competition in between hydroxyl-polyols and water for the reaction with an isocyanate group. The humidity content of the environment and the application temperature are restricting elements for polyurethanes and other chemically responding systems.
Hybrid systems currently have a larger scope for application conditions, however the existence of drivers in hybrids makes them more conscious humidity than “pure” polyurea systems. Moreover, because the catalyzed polyol/isocyanate reaction behaves in a different way from the amine/isocyanate response to altering application temperature levels, the system becomes less robust.
Polyurea can be utilized in severe conditions. When it is used on substrates almost filled with water, polyurea will not provoke blistering nor will blistering happen when the air consists of high quantities of humidity. Even at extremely low temperatures (as low as -20 deg C) the polyurea coating will still cure. Polyurea coatings integrate high flexibility with firmness. They are the most suitable coatings when the following is required:
high curing speed;
an application under high humidity and/or at low temperature levels;
extreme abrasion resistance;
impenetrable membranes;
high density develop;
chemical resistance.
Applications for Polyurea Coatings
A mutual understanding of the properties of polyurea spray coatings is needed to specify the ideal application. Table 1 offers a general introduction of the physical and chemical residential or commercial properties that can be anticipated of polyurea spray products. Polyurea systems are understood to be very tough. They integrate high flexibility with high surface firmness, resulting in great abrasion resistance.
The marketplace advancement started in the United States, followed by Asia, with very strong development throughout the second half of the 1990s. In the first stage of advancement, polyurea was used as a protective layer over polyurethane insulation foam for roof applications. In Europe, the polyurea spray coatings market only started to develop in the last couple of years.
The broad window of application conditions, with a high tolerance for humidity, both from the environment and from the substrate and temperature, makes polyurea a really appropriate coating for concrete in construction applications such as roofing repair work, containment liners, membranes, parking lot decks, bridges and offshore. The high abrasion resistance leads to its application in liners for truck, bulk transportation wagons, freighters and conveyor belts. Table 2 provides an introduction of the application fields where polyurea is chosen based upon several of its distinct application and/or movie residential or commercial properties.
Raw Materials
A polyurea spray coating formulation includes five various aspects: the isocyanate part; (reactive) diluent; polyetheramines; chain extenders; and ingredients, fillers and pigments.
Isocyanate
Because the most typically used isocyanate is diphenylmethane diisocyanate (MDI), this paper concentrates on MDI-based items. Aliphatic systems can be used where UV stability is a problem.
Standard polyurea spray coatings utilize MDI prepolymers with an NCO content of 15 to 16%. In this NCO variety, a great compromise between viscosity of the material and the reactivity of the system is gotten. Lower-NCO prepolymers have a greater viscosity, however give higher elasticity and slower reactivity. Higher-NCO prepolymers are lower in viscosity, which provides an efficient mix of the two elements. Nevertheless, they become a lot more reactive, with the danger of developing more internal tension. Higher-NCO prepolymers will be used if greater surface area firmness is required. Table 3 supplies an introduction of the primary properties of the MDI prepolymers used for polyurea spray coatings in Europe.
Diluent
Figure 3/ Carbamate Structure
JEFFSOL( r) PC, or propylene carbonate, is a reactive diluent for polyurea. Propylene carbonate has a high flash point, low toxicity and should not be considered as an unstable natural compound (VOC). The main advantages of using propylene carbonate are:
improved life span of the isocyanate-prepolymer;
a compatibilizer for the blending of the two elements in the mixing chamber of the spray gun;
a viscosity reducer for isocyanate-prepolymers;
improved leveling of the applied movie.
Propylene carbonate reacts with an amine to give a carbamate structure including a secondary hydroxyl group. Due to the quick response in between isocyanate and amine, the secondary hydroxyl does not have the opportunity to react with an isocyanate group. The propylene carbonate particle should, therefore, be considered as a mono-functional particle (Figure 3).
In applications where contact with water can not be avoided, using propylene carbonate ought to also be restricted, as propylene carbonate is entirely miscible with water, and unreacted propylene carbonate could be drawn out, increasing the water permeability of the film.
Huntsman Petrochemical Corp. owns specific patents relating to using propylene carbonate in polyurea elastomers. Other solvents or viscosity reducers can be utilized if they are compatible with the isocyanate component. They might be thought about as a VOC. However, they will increase the shrinkage impact.
Polyetheramines
The amine mix used in polyurea spray coatings is a mixture of polyetheramines and chain extenders. The primary part 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 main amine groups supply a really quick and reliable reaction with the NCO groups of the isocyanate component. Table 4 provides the properties of the polyetheramines frequently utilized in polyurea.
Chain Extenders
Diethyl-toluenediamine, or DETDA, is the basic chain extender utilized in aromatic polyurea spray coatings. DETDA adds to the difficult block and enhances the heat resistance of the cured movie. It is the most reactive amine in the resin mix but, because of the stage separation during the treating, it manages the reaction mechanism and makes it possible to spray a polyurea film.
Other chain extenders like 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 5 lists numerous chain extenders and their attributes. Considerably decreasing the response also suggests that the competition with the water reaction ends up being more important and precautions require to be taken.
Additives, Fillers, Pigments
Depending upon the application, solvents, additives, pigments and/or fillers are introduced to the formulation. Adhesion promoters like silanes are utilized to enhance the adhesion on steel and concrete. UV absorbers are utilized to decrease the yellowing result of fragrant polyurea systems. Fillers are added to lower the raw material cost and/or enhance the physical properties of the coating. The addition of pigment and/or fillers is limited since the viscosity of the two components at the application temperature level needs to be kept under control. Higher quantities of fillers and reinforcement fillers can be added to the system as a 3rd element.
Product-Application Specifics
The most essential element of managing polyurea coatings is the blending. Great mixing will be acquired in a suitable mixing module by impingement with mechanical purge. The operational pressure and temperature of the products will likewise help to enhance the mixing efficiency.
Due to the high treatment speed of polyurea and the brief mixing time, the items are blended by impingement at high pressure. Certainly, for field applications it is more effective to create the products on a repaired 1:1 volume-mixing ratio. The pressure utilized in the field will vary between 150 and 250 bar. The viscosity of the products at application temperature level ideally needs to be lower than 100 mPa.s, and the viscosity of the two elements needs to be at the very same level. The residential or commercial properties of these prepolymers can be found in Table 3. The viscosity of the resin mix at 25 deg C is roughly 900 mPa.s, dropping listed below 100 mPa.s at application temperature level.
Experiments prove that polyurea movies produced at 65 deg C, 70 deg C and 80 deg C have different properties, and these properties enhance with increasing temperature levels. The spraying devices has enhanced substantially. The brand-new spray equipment allows various temperature level settings for the two elements, ensuring an optimum mixing in the spray head. Other features are easier variable ratio settings; easy output control; and simple tracking of application specifications.
The index of a polyurea system is normally kept at a minor over-index of the isocyanate in the variety of 1.05-1.10. As the isocyanate group reacts to humidity, the excess isocyanate makes up for the ‘loss’ of isocyanate groups throughout storage and/or application. The film properties of the 1:1 volume ratio sprayed system were determined for an index variation in between 0.90 and 1.15. The test results suggest that the movie performs finest at an index of 1.05 and higher. Below an index of 1.05 the results can vary considerably and become unpredictable, even for little index shifts.
Elements of Spray Polyurea Technology
Polyurea application had some issues throughout the preliminary start-up phase, which are at the origin of the still-existing misunderstandings about polyurea technology. These issues can be associated partly to the absence of experience at the time of the technology introduction, partly due to the lack of sufficient application equipment, and partially to the reality that this new technology might not be applied in the same way as the existing coatings systems.
At first, polyurea spray coatings looked too easy to use. Polyurea is extremely fast – the coating can be put into service right away after the application, and the last properties of the coating are gotten only a few hours later on. Polyurea is not water- or temperature-sensitive, and is easy to develop and produce. The first systems on the marketplace were indeed very quick, with a gel time of less than two seconds, and initially a variety of problems were connected to the reactivity of the systems.
At first, substrate wetting was a problem. This problem was linked to the advancement stage of polyurea with using incredibly fast spray systems. Development programs focusing on adhesion on concrete, with polyurea systems presenting gel times of three to 4 seconds, resulted in cohesive adhesion failure in the concrete. In practice, to restrict the dangers under variable field conditions, a multi-layer system is used, made of a primer and a topcoat.
A second problem noted in the field was the lack of intercoat adhesion. Laboratory tests, with times in between coats of a number of weeks, have actually revealed that intercoat adhesion is great. When issues occur with intercoat adhesion, the majority of the time they can be related back to issues with the raw materials, the manufacturing of the systems or the spray equipment. Spray devices issues, or a disturbance of the feeding of one or both elements towards the mixing module, can trigger poor mixing. Adjusting the maker settings of the spray can resolve this.
Due to the high reactivity of the systems, the surface area quality of the sprayed movie was at first really bad. Fine-tuning the spraying equipment was an initial step towards resolving the problem. Making use of non-VOC reactive diluents and the advancement of brand-new MDI prepolymers with higher 2,4′- isomer material led to best surface quality without compromising on working time.
The cost of polyurea spray coatings technology is viewed as a barrier to entry. “Pure” polyurea systems are more pricey, when considering basic materials cost alone, however can be used in areas where all other systems will stop working or where they are not ideal. Also the initial financial investment in equipment is rather costly.
However, when estimating the capital cost for a task, polyurea is more competitive when both the processing time and the waiting duration before the layered substrate is returned into service are included.
As gone over above, the success of the job is very equipment- and applicator-dependent, and we believe that the high entry barrier can just ensure quality services from specialized and proficient operators.
Construction-Related Aspects
Polyurea spray coatings can be utilized under difficult weather due to the fact that the chemistry is extremely quick and they do not experience unfavorable adverse effects brought on by the existence of humidity. They likewise treat at temperature levels below 0 deg C. However, when using polyurea coatings, a variety of safety measures still need to be taken.
If guides are thought about, a great method is to start with the evaluation of existing primers with known performance on the substrate. It is important to determine the adhesion efficiency of the polyurea coating on the guides and to check whether the application conditions of the guide and the re-coating conditions for the primer still work for a system with polyurea as a surface.
Concrete Surface Defects and Surface Preparation
The low cost, high strength and structural homes of concrete make it the material of choice for the construction industry. Some of the common homes of concrete like the limited chemical resistance, dust release and porosity or permeability make it needed to put a protective and/or ornamental layer onto the surface.
The preparation of the surface area is incredibly important. Depending on the surface area quality of the concrete, one or more of the following actions requires to be taken:
water jet and/or solvent cleaning;
grit blasting;
bughole and crack filling;
repair work layer of concrete;
priming.
Steel Surface Defects and Surface Preparation
The life process of steel construction coatings depends mainly upon the protective system put in place. The life of the protective coating itself is strongly based on the surface condition prior to the application of the coating. The protection of the substrate is mainly acquired by ensuring a great adhesion. Two adhesion mechanisms are possible:
molecular destination of the interfacial forces from both the coating and the substrate,
mechanical bonding or anchoring of the coating on the substrate.
Depending upon the condition of the surface area, several of the following pre-treatments will be required: 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 area roughness SA 2 1/2 to SA 2 according to the specification ISO 8501-1, really 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
Blending Efficiency
The mixing efficiency of the application devices is of vital importance. When formulating a system or customizing an existing system, it is needed to validate the blending efficiency constantly. Figure 4 shows the impact of changing the blending on the physical residential or commercial properties of a developed item. The tensile strength almost doubles from 14 N/mm2 to 23 N/mm2, the angle tear boosts 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 residential or commercial properties is restricted.
Figure 5/ Physical Property Variations at Varying System Index
System Index Influence on Final Film Properties
Earlier experiments reveal that a polyurea coating requires to be created at an index above 1.00, indicating with a somewhat greater amount of isocyanate-groups than amine-groups. At an index of 1.00 or lower, the physical properties of the coating ended up being unreliable. Figure 5 shows that the majority of homes have great worths at indexes from 1.10 to 1.30. Above an index of 1.30 the efficiency tends to drop again.
Taking into account that, in practice, small variations might happen in the application criteria, related to the accuracy of the spray devices and variations in application conditions, it is more secure to operate 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 helpful for various reasons, such as a reduction in the raw material expense or enhancement in physical homes. Inorganic fillers have a various hardness and some will abrade parts of the spray devices more than others. The most delicate parts are the blending chamber and the nozzle of the spray gun. The filled systems require to be carefully filtered prior to packaging.
Depending on the performance of the spray installation, the dosing of filler can differ. We added up to 40% of filler to the resin mix. The main restricting aspect for the processing is the increase in viscosity of the filled part. This can lead to difficulties with the pumping system and/or bad mixing due to big differences in viscosity for both elements.
Including fillers improves the surface firmness, the angle tear and flexural bending modulus but has an unfavorable influence on the elongation and the tear propagation or trouser tear (Figure 6).
Figure 7/ Water Absorption for Different SUPRASEC Prepolymers
Water Absorption
For rust protection, the primary factor to consider for good efficiency is adhesion to the substrate. Further testing showed that, even with very good adhesion, the resistance to cathodic disbondment for some systems stops working. Given that cathodic disbondment occurs over a longer term and definitely is not an easy test approach, we determined the water absorption over a period of 10 days at 80 deg C and included 3% sodium chloride to the water.
As can be seen in Figure 7, the unmodified standard polyurea spray coatings offer only a restricted protection against corrosion. The water absorption drops significantly with increasing the NCO-content for the prepolymer. More fine-tuning of the pre-polymer led to water absorption being well listed below 0.5% after 10 days. Cathodic disbondment testing on this system offered really gratifying test outcomes.
Anti-Skid Performance
Flooring, car parks and sports floors are key applications for polyurea due to its high abrasion resistance, good mechanical homes and insensitivity to blistering during the treating procedure in humid conditions.
One example is the outside, impact-absorbing playground flooring. The system evaluated is based upon SUPRASEC 2049, formulated to be applied in a 1 to 1 volume ratio. The film properties are 70 Shore A solidity, 600% elongation and 13 N/mm2 tensile strength. The results in Table 6 program that it is completely possible to create a system, which abides by the antiskid needs of a flooring system, in this case for a flexible substrate.
Polyurea spray coating technology is various from other coating chemistries and can broaden the application series of coatings to locations and conditions where other coating systems will stop working.
Polyureas are very ideal for construction applications. The quick treating makes it possible to utilize when just very short disturbance periods are allowed. The fact that the isocyanate/water reaction is not impacting the physical properties of the applied movie, expands using polyurea to high relative humidity conditions and does not set such stringent limitations 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 basic materials for fine-tuning of the formulation and assessment of the system, in the often difficult conditions where the coating is to be used, is still needed.
Polyurea spray coating technology implies handling reactive chemicals. While handling the chemicals throughout their production, product packaging and application, the appropriate protective clothing ought to be worn at all times.
