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Plural-Component Equipment Essentials: Processing Polyurea Coating

MONDAY, JUNE 19, 2017

By Murph Mahaffey, Director of International Sales, PMC


Polyurea is an amazingly reactive technology requiring specialized equipment for application. The chemistry has been around since the 1980s, and as data has been acquired over time, both the chemistry and plural-component equipment required for application have matured. The most significant global growth of polyurea coating applications has been in the past 10 years, when polyurea technology has “come of age.”

The term “polyurea technology” refers to the process of coating with polyurea from beginning to end. Countless hours in lab and real-world testing have gone into developing and refining a very specific system of evaluating substrates and specifying applications, including surface preparation, use of primers, material formulation and processing requirements. Conceivably, at any hour of the day, anywhere in the world, polyurea is being applied by someone coating a substrate or caulking a joint. The breadth of work done with polyurea would not be possible without reliable, repeatable technology in place.

The PMC PH55/PHX-55 “Classic” is the company's highest output proportioner. PHOTOs: COURTESY OF PMC

SELECTION OPTIONS

Equipment performance is one of the most important issues in plural-component dispense technology, and equipment selection is critical. Here are some basic elements that determine if your equipment will have what it takes to process polyurea and maximize the physical properties of the system.

Application equipment is used to transition drum sets of material into a new floor, membrane or product. Polyurea technology demands a lot from this equipment — controlling proportion, pressurization, heat, mixing and dispensing.

Polyurea coatings can set in under 15 seconds. Precisely starting and stopping the flow of materials under pressure and heat requires more than just the right equipment. The application technician — who understands the equipment and is trained in its operation — makes the final application a success or failure. All of the research and development in the world won’t help a coating that is applied off-ratio or poorly mixed.

While there is a lot of information out there about application equipment, these basic questions should be asked to determine if it can reliably spray-apply a polyurea coating.

Direct impingement mixing: What is it?

Direct impingement mixing is the most popular method for fast-set, spray-applied polyurea. This is a buzzword in the industry that refers to the high-pressure combination and mixing of two material streams to develop a manageable spray pattern. Many material suppliers and formulators have agreed this type of mixing gives the best properties to polyurea coatings.

Where did this direct impingement technology originate?

History shows that impingement mixing was first used for spray-applying and pouring polyurethane foam. After the advent of polyurea chemistry, the industry-leading companies of the day continued to develop this technology, and direct impingement technology became the industry standard for polyurea spraying.

Now all of the equipment companies supplying to the polyurea industry offer a version of this technology, either as an add-on to old guns or as the heart of newly-designed dispense equipment. The AP-2 gun from PMC is an example of how this technology has been refined for the industry. To help with operator fatigue, the gun is lighter weight, and the triggering speed is increased by incorporating a short piston throw. Seals and screens are easy to access for cleaning and on-the-fly replacement.

The AP-2 air-purge gun
The PX-7 mechanical-purge gun

Which of the two types of spray gun — air-purge or mechanical-purge — should be used?

“Purge” refers to how the mix chamber or module will be cleaned after the gun trigger is released. Keep in mind that polyurea sets very quickly, so the mix chamber must be immediately cleaned to be ready to spray when the gun is triggered next.

AIR-PURGE GUNS, like the AP-2, are the most widely used type. Simple to operate and maintain, they will send a blast of air into the mix chamber and out of the nozzle immediately after the gun trigger is released.

MECHANICAL-PURGE GUNS, like the PMC PX-7, have a valve rod that pulls back when the gun is triggered, allowing the material to flow and mix. When the gun trigger is released, the valve rod moves forward, stopping the flow of materials and clearing the mix module of material.

Each type of gun when properly set up and maintained will spray polyurea successfully.

Pumping systems: What really works?

Dual-acting, positive displacement pumps are the standard for delivering the consistent spray pressure that is needed for polyurea coatings. This type of pump is moving materials on both strokes.

There is plenty of marketing hype about the orientation of the pump. Should it be horizontal or vertical? The truth is either type of pump will perform, but its orientation is only part of the equation and not the determining factor. Horizontal pumps have less pressure fluctuation when the pumps reverse and are, therefore, known for keeping a more consistent spray pressure.

The pumps are giving the correct proportion/ratio to the material and consistently pressurizing the two components for the best mix. The pumps need to provide the correct ratio of A and B chemicals and sufficient pressure for the materials to mix and dispense in a controllable way. The spray gun needs to be set up with the correct combination of mixing chamber/module and spray tip to make use of the pressure and ratio that the pumps generate. The pump system should be able to maintain the recommended mix pressure during spraying or when the system is “dynamic.” Look for is a consistent spray pattern that does not change or diminish when the pumps are reversing direction.

How much pressure?

The material supplier should always be consulted about the processing requirements for the polyurea being applied. The equipment should be able to maintain the pressure during the spray process. There are three terms used to describe pressure in spray equipment.

STATIC PRESSURE: The machine is pressurized and ready to spray but the gun is not triggered.

DYNAMIC PRESSURE: The gun is triggered, and the machine is spraying.

DYNAMIC CAPACITY: The machine is able to maintain a specific spray pressure at a specific flow rate.

Many polyureas today are sprayed in the 1,800 – 2,200 psi range (124 – 151 bar). Some formulators may specify more or less pressure, but this is a general range.

Pressure drop is the difference between static and dynamic pressures. Again, it is imperative that the dynamic pressure of the machine can maintain the spray pressure recommended by the material supplier.

Hydraulic, pneumatic or electric drive?

At the end of the day, each type of pump drive can deliver a quality product. Longevity and consistency are what set them apart. Here are key differences to consider:

  1. Hydraulic drive equipment is the workhorse of the industry and the most reliable.
  2. Air-driven equipment typically costs less because it does not have a hydraulic power pack, but it does require a large compressor for operation. Air motors can be prone to freezing problems when running a long time for larger jobs in hot and humid conditions.
  3. Electric drive machines should be monitored as the motor warms up. The resistance will change, which may require adjusting the spray pressure during a job.

Selection of the drive often has to do with what is already in place at a business; many companies prefer the more reliable setup and lower air-volume requirement of the hydraulic machine. Some companies already have large air compressors and go with air-drive equipment.

With either type of drive, the main requirement is consistent operation and sufficient pressure supply to the material pumps. The drive maintains the dynamic pressure required to mix the A and B components and is a crucial component for coatings success.

How much heat?

Direct-contact, digitally controlled hybrid heaters are currently used for transferring heat to polyurea components. We need heat to lower the viscosity of the materials and facilitate the most complete mix. Material suppliers should be able to recommend a processing temperature for their materials.

WATTAGE is touted as the main indicator of heater performance and is very important. It should be noted that wattage is a function of the amps and volts that are fed to the heaters. Even though a heating system is rated at 12,000 watts, it may only be achieving a portion of its actual rating depending on the measured incoming voltage.

SURFACE AREA is the other key factor in determining the efficiency of the heater. Having a large surface area on the heating elements will determine how long the materials are exposed to heat before entering the heated hose. This is called the dwell time.

The amount of dwell time in the heater has a direct effect on the Delta T. Delta T describes how much the temperature of the materials can increase at a specific flow rate. The heater wattage and surface area will determine the Delta T, or amount of heat that the machine can transfer to the materials while dynamic.

CONCLUSION

Spray equipment for polyurea provides ratio, heat, pressure and mix to create a controllable spray pattern and allow for the physical properties of the polyurea coating to be realized. Polyurea is a reliable technology, but only as reliable as the machine and materials used. To ensure job success, a little research prior to equipment selection can guarantee that the best properties of the coating will be achieved.

Companies dedicated to the design and manufacturing of polyurea and polyurethane equipment work to ensure that advances will continue in dispense technology. Watch this page for news on polyurea use and innovation.

*Claims or positions expressed by sponsoring authors do not necessarily reflect the views of TPC, PaintSquare or its editors.

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ABOUT THE AUTHOR
Murph Mahaffey, Director of International Sales, PMC

Murph Mahaffey holds a B.A. in business anthropology and specializes in the development of mutually beneficial international business relationships. He has years of international direct/distribution sales channel development experience, including product identification and market development, as well as extensive experience driving sales success in Europe, Asia-Pacific, the USA, Canada and South America. He is a frequent presenter for industry associations, including SSPC, PDA and PDA-Europe.

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