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Powder particle size: Its effects on coating line performance Paul R. Horinka Morton International, Powder Coatings This article is the first in a three-part series on the role powder particle size plays in the handling and perfor- mance of powder coatings. The article discusses powder particle size in relation to powder handling, charging, delivery, film characteristics, equipment, and the coat- ing process. Subsequent articles will discuss the role of particle size in electrostatic charging, and particle size measurement and instrumentation. Although particle size is not the only influence affecting a powder coating’s pe$ormance, a better understanding of it will help you gain control over your powder coating operation. owder coatings are made up of an assortment of relatively small particles produced by a grinding P and collection process. The size of these particles can have a major influence on many properties related to powder handling, charging, delivery, and final film characteristics. For these reasons particle size is an important factor in initiai product design and selection. Powder coatings manufacturers may offer stock prod- ucts that are created to meet a variety of needs. They may also tailor products by particle size to meet speci- fied film thickness requirements, part configurations, and certain application conditions. The coater also plays an important role in on-line parti- cle size control in situations in which powder is reclaimed and reused. Understanding the role of particle size in the electrostaticspray process can serve as a tool for system design, control, and troubleshooting. All of these can help achieve cost savings through improved efficiencies. Particle size in relation to powder handling Steady, even fluidization produces consistent powder flow through pumps, transfer hoses, and spray guns. As the concentration of fine particles increases, powder tends to pack and become difficult to fluidize. The fine particles fill the void spaces between the large particles. As a result, higher-than-normal air pressures are required to move the compacted powder. Air passing through the powder in the hopper finds the path of least resistance and geysering occurs. Geysers, large air bubbles, and dead spots in the fluid- bed hopper cause air surges and delivery of inconsistent amounts of powder to the pickup tube. Although pumps are designed to disperse the powder in air, they don’t prevent air surges from making their way to the spray gun. As a result, spurts of powder are easily visible as puffs in the powder cloud. In minor cases, these puffs, or gun spurts, produce uneven film thickness; in major cases, they create blotchy mounds of powder on the Parts- The higher-than-normal air pressures required to move compacted, fines-laden powder frequently cause impact fusion, which can occur at any contact point with the powder stream but is most prevalent in bends, turns, and the deflector or the gun tip. The grinding action of high pressures can also cause the powder to break down in the system, generating even more fines. This eventu- ally leads to a complete system shutdown because of the inability to move powder. At this point, the only option is to clean up, dump all powder from the system, and recharge with virgin material.

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POWDER COATING, February 1995 37

Powder particle size: Its effects on coating line performance

Paul R. Horinka Morton International, Powder Coatings

This article is the first in a three-part series on the role powder particle size plays in the handling and perfor- mance of powder coatings. The article discusses powder particle size in relation to powder handling, charging, delivery, film characteristics, equipment, and the coat- ing process. Subsequent articles will discuss the role of particle size in electrostatic charging, and particle size measurement and instrumentation. Although particle size is not the only influence affecting a powder coating’s pe$ormance, a better understanding of it will help you gain control over your powder coating operation.

owder coatings are made up of an assortment of relatively small particles produced by a grinding P and collection process. The size of these particles

can have a major influence on many properties related to powder handling, charging, delivery, and final film characteristics. For these reasons particle size is an important factor in initiai product design and selection.

Powder coatings manufacturers may offer stock prod- ucts that are created to meet a variety of needs. They may also tailor products by particle size to meet speci- fied film thickness requirements, part configurations, and certain application conditions.

The coater also plays an important role in on-line parti- cle size control in situations in which powder is reclaimed and reused. Understanding the role of particle size in the electrostatic spray process can serve as a tool for system design, control, and troubleshooting. All of these can help achieve cost savings through improved efficiencies.

Particle size in relation to powder handling Steady, even fluidization produces consistent powder flow through pumps, transfer hoses, and spray guns. As the concentration of fine particles increases, powder tends to pack and become difficult to fluidize. The fine particles fill the void spaces between the large particles. As a result, higher-than-normal air pressures are required to move the compacted powder. Air passing through the powder in the hopper finds the path of least resistance and geysering occurs.

Geysers, large air bubbles, and dead spots in the fluid- bed hopper cause air surges and delivery of inconsistent amounts of powder to the pickup tube. Although pumps are designed to disperse the powder in air, they don’t prevent air surges from making their way to the spray gun. As a result, spurts of powder are easily visible as puffs in the powder cloud. In minor cases, these puffs, or gun spurts, produce uneven film thickness; in major cases, they create blotchy mounds of powder on the Parts-

The higher-than-normal air pressures required to move compacted, fines-laden powder frequently cause impact fusion, which can occur at any contact point with the powder stream but is most prevalent in bends, turns, and the deflector or the gun tip. The grinding action of high pressures can also cause the powder to break down in the system, generating even more fines. This eventu- ally leads to a complete system shutdown because of the inability to move powder. At this point, the only option is to clean up, dump all powder from the system, and recharge with virgin material.

-

38 POWDER COATING, February 1995

Fine particles also have more surface area per given weight than large particles. Consequently, fine particles pick up more moisture than large particles, leading to particle agglomeration, often referred to as lumping, or clumping. These lumps must be broken up by an in-line sieve to avoid another source of gun spurts. Lumps often occur as powder sits in an unused hopper for a time, pos- sibly as short as overnight in hot, humid conditions. In these cases, hand agitation and a few hours of fluidiza- tion with dry air drives off the moisture to temporarily ease the symptoms. This doesn’t address the root cause of high fines content, however.

Particle size in relation to powder charging The theoretical role of particle size in electrostatic charg- ing is well documented and will be the subject of a future article in this series. The most important facts relating to line operation and performance can be summarized by saying that small particles tend to charge more effi- ciently than large particles. This leads to a number of important effects often associated with electrostatic spray application.

Wraparound. Perhaps the most widely recognized phe- nomenon seen in powder coating is wraparound. This occurs partially because powder particles follow electro- static field lines to the electrically grounded part. Concentrated on external edges, the field lines bend around the part to engulf it. Although air currents also affect this property, the absence of fines dramatically decreases the wrap.

The wrap decreases because the airstream carries fine particles more readily than it carries large particles and the relatively weak electrostatic force influences fine particles more easily than it influences large ones. When compared with fine particles, large particles tend to have more staight-line motion. In addition, gravity affects large particles more readily than it affects fine particles.

Powder films are self-limiting in coating thickness because of the charge buildup in the powder layer. Once an upper limit in charge is reached, the powder no longer deposits efficiently. Because small particles carry more charge per unit of weight than large particles, they become self-limiting at a lower film thickness than that of large particles. The phenomenon, known as back ion- ization, occurs at this self-limiting point.

Faraday cage effect. Faraday cage penetration prob- lems follow the same line of reasoning as wraparound problems. The simplest example of a Faraday cage is a box. Powder preferentially deposits on sharp edges or external corners. These corners are the first to attract powder and become self-limiting. The deposited powder itself produces an image field that helps to repel oncom- ing powder.

‘ 1 7

In addition, conductive parts shield the inside corners from the electrostatic field produced by the guns. The air carrying the powder swirls inside the Faraday-cage area and also sweeps powder away. Small particles, because they deposit on edges more readily and become self-limiting faster than large particles, create more difficulties in penetration than large particles do. Large particles, because they are slower to self-limit and have truer straight-line motion than small parti- cles, are more successful in penetrating corners than small particles.

-

Fines buildup. When film builds start to decrease and coverage in corners becomes difficult, one fi-equent cause is a buildup of fines in the coating system (another is poor electrical ground).

Particle size in relation to powder delivery Fine powders can create problems in the powder room. They can generate dust in transfer operations or during powder adds. Fine powders also tend to drift out of the powder booth more readily than other powders if designed airflows at the booth openings are not main- tained or if strong drafts exist outside the booth. Powder dusting and drifting can be a major source of rejects because of contamination.

As mentioned above, aerodynamics combined with elec- trostatics are important variables influencing penetra- tion and wraparound. These two factors also control delivery of the powder to the part. Generally, air drives the particle to the part. Once the powder is in close prox- imity to the part, however, electrostatic attraction takes control. The success rate in delivering powder to the part is known as transfer efficiency. In other words, transfer efficiency is the portion of powder deposited on parts compared with the total amount of powder sprayed from the guns. It is expressed as a percent value.

At one time, the industry viewed transfer efficiency as an aflerthought in the powder coating process. It dis- missed its relevance because overspray was collected and reused, thearetically giving 100 percent material use. Now, the industry recognizes that continued recy- cling of powder can have a negative impact on the final product for the following reasons:

.Particle size degradation and classification

Contaminants introduction

Segregation (of metallic-rich components for example)

.Scrap powder production (which must be discarded)

Even though small particles charge efficiently, they tend to get caught up in gun or collector airstreams, resulting in poor transfer efficiency. If particles are too large, they

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are prone to fall to the booth floor as a result of gravity and the momentum supplied by the conveying air (see Figure 1). A buildup of either the smallest or largest par- ticles from the starting, or virgin, powder distribution negatively impacts the powder’s reclaimability.

Particle size in relation to film characteristics

Particle size affects the film thickness achievable in the electrostatic spray process through the self-limiting mechanism. An overall coarse size distribution produces thick film builds; a h e size distribution produces thin film builds.

Because powder particles must melt, flow, and coalesce to form a film, the initial size of the particle impacts film smoothness. Large particles simply take longer to melt than small particles. As a result, large particles can form macroscopic hills in the film, causing orange peel.

Orange peel can also result from other factors, includ- ing coating rheology, surface tension, and the electro- static process itself. Of these, only surface tension doesn’t have a strong particle size connection. Powders with low melt flow have surface roughness approaching

a textured finish if large particles are present. When such powders are applied at 2 mils or less, seeds or pro- trusions interrupt the surface.

Once a coating reaches the self-limiting point, back ion- ization causes surface disruptions in the powder layer. These tiny explosions dislodge powder, leaving a surface defect upon cure. When this phenomenon occurs on a large scale, it’s responsible for what is called electrostut- ic orange peel, which is more prevalent with h e pow- ders than with other powders.

Particle size in relation to application and recovery equipment

Much of the preceding discussion has focused on pow- der particle size as the powder is delivered from the manufacturer. It’s also crucial to understand that the application equipment-+specially the powder recovery system-can dramatically affect on-line particle size change.

Manufacturers design reclaim systems to remove pow- der particles from an airstream. The systems do so with varying efficiency based upon particle size and are therefore particle size classifiers. Most powder coaters use application equipment with cyclone and cartridge recovery systems.

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Cyclone recovery systems. Cyclone systems depend upon a vortex of swirling air and gravity to knock down heavy or large particles into a collection vessel. The light, small particles stay entrained in the airstream to be ultimately removed by some sort of final filter, such as a bag house. (A typical cyclone recovery system is shown in Figure 2.)

As a result, cyclone systems tend to remove fines and build up coarse particles. Continued passes through the system, however, produce a grinding action that can regenerate some fines (see F'igure 3).

A cyclone recovery system (the small arrows indicate the direction of airflow in the system)

Powder booth Cartridge collector

I Pinch valve Cyclone Sieve I Filter

I c- ' Hopper

Clean air to

Final filter

Blower motor

C a r t h , a recovery systems. These systems depend upon a nit-hanical filter to separate powder from the airstream (st 3 Figure 4). As a cake of powder builds up, these filters become very efficient at removing it, send- ing particles of all sizes back to the feed hopper. With continued recycling, cartridge systems tend to build up fines in the reclaimed powder (see Figure 5).

To overcome these classification processes, attention to the following two factors is required. Both fwAms have a major impact on consistent operating pel-formance because they can dramatically affect particle size on line.

~

Control of virgin-to-reclaim ratio in the powder feed hopper

*Thorough system maintenance to assure that the sys- tem is operating properly

You might think that an in-line sieve would also be an important piece of equipment when discussing particle size. Its role, however, is simply to provide protection from outside debris pulled into the coating system and agglomerated powder, from impact fbsion for example, by removing extremely large pieces of trash. An in-line sieve isn't intended to be a major modifier of particle size.

Distribution of powder particle size: virgin versus cyclone scrap 12

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591.99 296.00 148.00 74.00 37.00 18.50 9.25 4.62 2.31 1.16

Microns

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Particle size in relation to the coating process

Coating factors such as transfer efficiency, virgin-to- reclaim ratio, and system maintenance are just as important as the particle size built into the product by the powder manufacturer or the powder application equipment used. Particle size is therefore a dynamic sys,

A cartridge recovery system

Powder High-voltage Collector with spray gun power supply color module

tem that must be managed on the coating line. To accomplish this, it’s necessary to maximize transfer effi- ciency. As a result, you decrease the amount of powder put into reclaim. Following are some fadors that lead to poor transfer efficiency:

Poor electrical ground

*High air pressures; high powder output from guns

*Poor part presentation, including gun aiming and part racking, spacing, and batching

*Poorly matched gun output to line speed

*Excessive manual touch-up

*Poorly maintained equipment; broken guns

Once the transfer efficiency exceeds 50 percent, you can address the virgin-to-reclaim ratio. If the system is fitted with an automatic reclaim transfer system, set it to maintain greater than 50 percent virgin powder in the feed hopper. Once the amount of reclaim exceeds the vir- gin powder in the feed hopper, the system begins to change in particle size as outlined above. This becomes a

Distribution of powder particle size: virgin versus cartridge hopper

591 99 29600 14800 74 00 3700 1850 9 25 4 62 2 31 116

Microns

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trend that is difficult to reverse. Manual transfers should be frequent and timed to correspond with virgin powder use. To control coating line performance, it’s essential to maintain a high virgin-to-reclaim ratio.

System maintenance items in addition to those men- tioned above include the following:

.Repair leaks in cyclone system duct work (also check seal at reclaim canister)

.Empty cyclone collection drum to avoid overfilling

.Maintain blower fans to specified airflows

.Replace torn or plugged filter cartridges as indicated by Magnehelic gauge’ readings

S u ” w Particle size is an important, fundamental property of powder. It directly affects the quality of coatings pro- duced from an automated powder line. Managing parti- cle size change on line requires planning, cooperation of other departments outside the powder coating shop, knowledgeable coating and equipment suppliers, and dedication to practices such as training and preventive maintenance. The benefits, however, include control

Pc over your coating line and consistent quality.

Endnote 1. Magnehelic gauge. A trade name for a differential pressure gauge that

measures pressure drop across the filters.

References M.A. Fooksman, “Comparison of Recovery Systems,” Powder Coating, vol. 2, no. 1 (February 1991), 37-44.

J. F. Hughes, Electrostatic Powder Coating (New York, N.Y.: John Wiley and Sons, 1984).

H.J. Lader, “Particle Size Modeling of Powder Paint in a Recovery System” in Powder Coating ’94 Proceedings (Alexandria, Va.: Powder Coating Institute, October 1994, 254-267).

D.C. Tyler, “Powder Particle Size Matters,” Products Finishing (January 1990), 66-73.

Paul R. Horinka is senior applications engineer at Morton Intermtional, Powder Coatings, 150 Columbia Ave., Reading, PA 19601; 6101372-3600, ext. 7689. He holds a BS degree in physical science and chemistry from Pennsylvania State University, University Park, Pa. A member of the American Chemical Society, AFPISME, and the Electrostatic Society ofAmerica, he holds several patents, including one specifically dealing with the affect ofparticle size on an application problem.

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