The following explanation can quickly help any contractor/applicator to understand the polyurea application process, the interplay between the various pieces of equipment, and the important design considerations of the pump, hoses and spray gun. This knowledge can help open the door to a whole new market for coatings companies that wish to grow their business.
The Newest Frontier in CoatingsPolyurea has become increasingly popular for many applications formerly dominated by epoxy, acrylic and polyurethane. Specifically, it is the aliphatic (an open-chain organic compound) polyurea compounds that increasingly find themselves as the coating of choice due to their excellent mechanical properties (up to 3000-psi tensile strength and 250- to 500-psi burst strength); low-temp flexibility (even down to –40 C); UV stability (color durability superior to polyurethanes); and low volatility (due to 100 percent solids formation).
Capable of smoothly adhering to substrates like cement, brick and fiberglass, polyureas are now used for scores of building applications such as encapsulating asbestos ceilings or coating floors. Unlike polyurethanes, polyureas cure under conditions of high moisture and humidity, as long as the correct surface preparation is undertaken.
Within manufacturing and process industries, polyureas now find use in everything from the lining of storage tanks to covering shop floors and plant walls. Polyureas vastly outperform paint, which often cannot hold up to rigorous cleaning procedures practiced in pharmaceutical and food processing plants, for example. The extremely fast reaction time (three to 10 seconds) of polyurea systems appeals to facility management as it permits quick installation with only minimal disruption to process operations.
These performance benefits explain, in part, why the booming demand for polyurea systems should continue well into the future. “From what we’ve learned as an association, in addition to my interactions with applicators in the field and companies that manufacture the systems and the raw materials, I see a 15 to 20 percent growth per year in this industry,” says Dudley Primeaux, former president of the Polyurea Development Association and owner of Primeaux Associates LLC of Austin, Texas. As the prime inventor of sprayed-polyurea systems, Primeaux is well positioned to provide consulting advice to contractors interested in entering the field.
A Little Background Knowledge Goes a Long WayPolyurea systems are formed from the combination of two components: isocyanate and an amine resin. The union of these components forms a urea linkage that is highly flexible, unlike the crystalline nature of polyurethane systems. The initial formulations were introduced in 1986 when somebody told Primeaux, then a chemist with Texaco Chemical Co./Huntsman Chemical, that polyureas couldn’t be sprayed. He accepted the challenge.
As Primeaux likes to relate: “I didn’t know any better. So I took it upon myself to see what was possible. I needed the equipment first to see what, if any, formulations could be developed, so I called the first equipment manufacturer I could locate, which was Gusmer. I asked for the biggest two-part spray/delivery machine they made.” Founded 1961, Lakewood, New Jersey-based Gusmer Corp. has long been a leading supplier of plural-component proportioning equipment for spraying elastomeric coatings.
“Once we received the equipment, the system worked the first time because the equipment delivered both the A and B components in a predictable, consistent manner,” continues Primeaux. With the equipment working properly, “It gave us the opportunity to perfect improving the resin formulations.”
The first commercial sprayed-polyurea coating was delivered for a roofing system in 1989, and the history of coatings systems has been forever changed. But the polyureas provide optimum performance only when mixed correctly, and according to Primeaux, choosing the right equipment to handle this task is critical.
The Challenge of Getting the Right MixNo chemical catalyst is required, but due to the extremely fast reactivity and cure, special plural equipment is required. High pressure is absolutely necessary to force the components to mix. High temperature is also needed to lower the viscosity to enhance the mix and atomization. Heating takes place at the pump and the hoses, while the actual mixing takes place inside of the gun where the two components meet at high velocity. This mechanical mixing is critical to uniform concentration (hence, better adherence to the substrate). The importance of obtaining equipment specifically designed to spray polyureas cannot be over-stressed.
“The quality and the tolerances in the manufacturing of the spraying equipment make a huge impact in the output of your product,” stresses Primeaux. “Good equipment costs more, but this is not the place to skimp when you want to spray polyurea. The key to processing is within the proportioning pump and the spray gun. This is the ‘life support’ system for proper installation and application.”
It All Begins at the PumpWhen spraying with polyureas, special consideration starts with the selection of a high-output pump. Increased pressure delivers more kinetic energy to the mixing zone. Pumps can be either pneumatic or hydraulically operated. Both types still require an air compressor because the air drives the drum pumps that deliver the material. The choice in pumps is then divided into either a vertical or horizontal layout.
The vertical pump has been the traditional choice of paint contractors. However, with high-solids coating systems like polyureas, verticals will not fill components of different viscosity at the same time and rate of speed. An imbalance of pressure will always be noted between the up and down stroke. This often results in a pulsating flow of material to the spray gun and affects the spray pattern as well as the coating consistency.
Polyureas must be mixed with an even and strong head of pressure. If the pressure varies on one side or the other, the quality of the mixed material and the consistency of the spray pattern at the gun can be adversely affected. And without sufficient volume, poor mix, atomization and application result.
HosesThe hoses that carry the components to the gun must also be heated; otherwise all the gains in breaking down the viscosity at the pump would be lost. Regular hoses, designed for polyurethanes, cannot handle the higher temperatures and pressures required for spraying polyureas. While the initial heating takes place within the pump, the hoses must maintain that temperature throughout their entire length — even as long as a football field. Such demands require specially designed hoses.
Primeaux added that, depending on the length, hoses should include a “step-down” in inside diameter (ID) so as to minimize pressure drop at the spray gun. In other words, the ID at the pump end of the hoses is larger than the hose section near the gun.
The Spray GunGuns are basically divided into two types — mechanical and air depending on how the material is purged out of the chamber when detriggering takes place. Solvent purge guns are also occasionally utilized for hard-to-mix or off-ratio spray applications.
Representing the most applicable spray gun for the application of polyurea technology is the mechanical purge spray gun. With a mechanical gun, the return of the valving rod at detriggering seals off and completely flushes out the mixing chamber area. The tight fit of the valving rod, along with the high-pressure kinetic force of its movement, cleans all unwanted material out.
“The mechanical purge gun gives the best mix and properties, partly because of the dynamics within the mixing chamber,” says Primeaux. “You also get a complete purge out of the mixing chamber, which eliminates the possibility of hold-up of product inside that chamber. The problem with hold-up is that it affects the incoming material and the subsequent mix.”
With air purge guns, a blast of air actually blows the material out upon detriggering between sprays. Here, the valving rod moves back and forth to allow air to enter the mixing chamber, as opposed to just material.
The shortcoming of air purge guns is that since air is used to purge, the pressure must be consistently high to completely flush the chamber. If not, the tip will become plugged. An additional problem occurs whenever air enters into the mixing chamber. Contaminants such as oil and water could be introduced from the air compressor onto the substrate. This can cause blistering and delamination of the polyurea coating.
Additional Gun ConsiderationsOther considerations when choosing a gun are gun geometry and construction. Maintenance and speed of operation also rank as important factors when selecting polyurea spray systems. Operator ergonomics should be taken into account as well. Finally, contractors new to the polyurea spraying industry should also seek out an equipment manufacturer that provides good support and a commitment to maintaining a ready inventory of spare parts.
Do the Job Right the First TimeUltimately, contractors reap the benefits of knowing which equipment to use for entering the field of sprayed polyureas. On the other hand, the consequences of ignorance can put a contractor out of business.
“When you’re working in the trenches with these applicators like I do, you can see the results of using poor equipment right up front bam, there it is!” explains Primeaux. “I know of one case where a guy did a job for a customer, but two things went wrong. The contractor didn’t do the right surface prep and the equipment was not really suitable for the field application of polyurea. When the coating failed, the customer sued the contractor for several times what the job was worth because of lost revenue.”