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Cleaning & Rinsing for Powder Coating

by Bob Utech
Powder Visions - Brooklyn Park, MN, USA


Here's a little information I will give you from my upcoming new book...

Generally the rinse is critically important in pretreatment. Rinses are utilized to rinse prior applied chemicals and rinse any residual contamination that may remain on the part and help neutralize chemicals. Rinsing with city water will almost always leave your substrate with total dissolved solids (TDS) remaining.

Rinsing is a critical step in powder coat pretreatment because a part is no cleaner than the quality of rinse water used.

Surface cleanliness is a fundamental prerequisite in quality painting. Contaminates interfere with adhesive forces that bond paint to a surface. This is true no matter what surface-metal, plastic or wood.

Various processes are used to clean substrate surfaces, including aqueous, solvent, abrasive, flame and cryogenic. The optimum process depends on many parameters, including the type of material being finished the size and shape of the object, the end-use environment, the desired life expectancy of the applied coating and the type of coating.

Aqueous cleaning is by far the most common. Aqueous cleaning systems are most often power spray washers. These washers include four basic processes: cleaning, conversion coating, sealing and rinsing. The function of the cleaning is to remove soils, oils and other contaminates. The purpose of the conversion coating is to alter the surface chemically with a material such as a phosphate to improve corrosion resistance and paint bonding. The goal of sealing is to finesse the conversion coating and give the surface the proper pH for accepting paint. The task of rinsing is to remove dragout contaminates between stages and after the last chemical stage.

Which of the four aqueous processes is the most important? Each is equally vital. Each process can be likened to four legs on a table. Remove any leg, and the table will fall. Take away any aqueous surface preparation process, and the end result will not provide optimal paint bonding.

Rinsing tends to be taken for granted because it is such an apparently simple process involving only water. However, water is a very complex chemical. Put all three together: taking rinsing for granted, its apparent simplicity and water's chemical complexity, and this leaves a process that is actually little understood.

It is water's strong hydrogen bonding that gives it a high surface tension. Water's surface tension can be demonstrated by floating a needle on the surface of water. The hydrogen bonding at the surface prevents the needle from sinking, even though the needle has about seven times the density of water.

Hydrogen bonding is responsible for many of water's properties, such as surface tension and viscosity. Both surface tension and viscosity decrease as the temperature of the water increases. This is because increased molecular motion decreases the strength of the hydrogen bonds.

Water's chemical structure makes it an ideal solvent. Pure water has a very low conductivity. However, as the ionic content of water increases, the conductivity will increase. Therefore, conductivity is a good measure of the purity or amount of dissolved solids in a water sample.

The ability of water to wet and properly rinse a surface is intimately related to water's chemistry, especially hydrogen bonding and polarity. A raindrop is a sphere because of the hydrogen bonding between neighboring water molecules. Each water molecule is bonded to another.

A drop of water resting on a surface is a different story. The effect of water molecules on each other is not the sole factor to be considered. Some hydrogen bonding, or dipole polarity effects, comes into effect at the water-surface interface. The amount of this interaction depends on the polarity of the surface. A surface containing a high degree of polarity will spread the water evenly, leaving no sign of a spherical drop. A non-polar surface will reject interaction with water, and a drop of water will tend to retain a spherical shape.

A clean surface is said to be hydrophilic (water loving); an oily surface, hydrophobic (water hating).

This is why water will bead up on a waxed car surface. To the water drop, the waxed surface appears soiled, oily, passive or non-polar. Hydrogen bonding occurs within the water drop instead of to the surface.

A majority of finishers make several mistakes that lead to overuse of water and an increase in pretreatment chemistry costs. Following is a list of common mistakes finishers make:

Water the problem with purity

In its pure state, water is one of the most aggressive solvents known. Called the universal solvent, water will, to a certain degree, dissolve virtually everything to which it is exposed. Pure water has a high-energy stage, and like everything else in nature, wants to achieve energy equilibrium with its surroundings. It will attempt to dissolve the quantity of material required to reach saturation, which is the point at which no higher level of solids can be dissolved. Contaminants found in water include atmospheric gases, minerals and organic materials from the earth (some naturally occurring, others man-made) plus any materials used to transport or store water.

The following data will help you decide if you require a spot free rinse. If you do, it should also help determine which system is best suited for you in order to generate spot free water.

First, what are spots? Spotting is the residue of dissolved solids that are left behind when a water droplet evaporates. The higher the total dissolved solids (TDS) are in the water, the worse the spotting. How? As water stops sheeting (running) off a surface it forms little half-moon shapes in a process we commonly refer to as beading up. (It technically is the formation of a meniscus, which has to do with surface tension and wetting ability). If you think of that bubble as a small mountain and remember how gravity works the way a spot forms is obvious. So is the reason it always looks like a small donut so to speak. As the bubble evaporates the solids (which don?t evaporate) the solids settle out in the shape of the bottom of the bubble. Since many of these solids are actually salts, it also becomes obvious why soft water will often spot more than hard water, since softening, after all, merely replaces metallic ions with sodium (salt) ions. This is generally why water softening alone probably should not be used for pretreatment in powder operations. We don't need the sodium ions on the parts nor do we need the spotting received from the sodium ions. Check with your chemical supplier to get a water analysis to know for sure.

How much (many) of these total dissolved solids do you need to have spotting? At about 40 to 50ppm (parts per million) you will see spots on dark parts. At about 75ppm you will see spotting on glass and chrome and at about 150ppm you will see spotting on all surfaces.

What is the normal TDS of water? According to some recent trade journals, we average between 250 and 300ppm TDS in the United States. That means that in the average water supply in the United States, if you require a spot free part, you will have to treat the water.

How? There are two basic methods of removing solids from water, Deionization (DI) and Reverse Osmosis (RO). Deionization is much like a water softener where water is allowed to flow through a resin bed, which absorbs out all the solids. The exhausted resin bed is exchanged with a fresh tank/s by the Service Company who charges for each exchange.

In an RO unit, the water is forced through a membrane, which literally filters out the solids.

One of the factors you should know about is the quality of the incoming raw water. Have an independent source or your chemical pretreatment vendor analyze your water and explain why its constituents behave in a positive or negative way. By understanding this resource, you can elect to treat the incoming water in one of three ways.

Softening:

This method exchanges high amounts of calcium, magnesium, or other minerals found in your water for sodium. A common industrial sized softener can be used to remove those water constituents that lead to scale build up in the nozzles, the tank walls, and the heating apparatus found in heated washers. Sodium is more is more soluble and less likely to produce hard scale than the minerals it replaces. If you have hard water and use alkaline cleaning baths, you are a good candidate for softened water, which is typically in a hardness range of 15 grains per gallon or more.

However, the exchange of calcium and magnesium for sodium raises the TDS level. This rise in TDS is typically due to lack of adequate or complete back flushing when removing excess sodium. Make sure your softener is well designed, sized and maintained.

(NOTE!) A softener is usually not recommended for the rinse preceding and following the phosphate stage and for final seal-rinse stages because the dissolved solids left behind are more soluble and conducive to corrosion than the original hard water minerals.

Reverse Osmosis:

This form of water conditioning can produce high-quality water for finishing. Basically, reverse osmosis is a process in which water is passed between semipermeable membranes. These membranes can be designed to remove hardness, minerals and other constituents that interfere with high-quality pretreatment.

Reverse osmosis systems tend to be most desirable when large volumes of an improved water source are necessary. In many cases, a blend of reverse osmosis and raw city water is used to improve the water for active chemical stages.

Reverse osmosis systems are generally more expensive than deionized systems initially to install, but are much cheaper to maintain once installed.

Deionizing:

Basically, deionizing water relies on reactions. The first reaction uses a cation-exchange resin regenerated with an acid to remove all metal ions and replace them with hydrogen ions. The second reaction is an anion exchange to remove the acids produced. This reaction is regenerated with an alkaline solution.

By definition DI water has had all of the cations (positive ions) and anions (negative ions) removed and is water in the pure state. However, this isn?t true in reality because of the nature of the deionization process. DI water usually has been run through cation and anion exchangers. Most cation exchangers work more efficiently than anion exchangers do. This means that DI water will actually contain a small percent of excess anions and, therefore, be slightly acidic.

Deionized water is the most frequently used source of high-quality water in the final rinse stages of surface preparation. This step removes unreacted deposits and leaves the substrate virtually free of dissolved and undissolved solids.

All naturally occurring water contains dissolved mineral salts. In solution, salts separate into positively charged cations and negatively charged anions. Deionization can reduce the amounts of these ions to very low levels through the process of ion exchange.

Cations are removed by cation exchange resin. It replaces sodium, calcium, magnesium, and other cations with hydrogen ions (OH). The exchange produces acids, which must be removed or neutralized by anion exchange resin.

Two types of anion resin are used for deionization: weak base resin and strong base resin. Weak base resin absorbs strong acids, while strong base resin exchanges chloride, sulfate, and alkaline anions for hydroxide ions (OH). The hydrogen ions from the cation exchange process combine with the hydroxide ions from the anion exchange process to form water (HOT or H2O).

Because the deionization process is so effective, the water quality is usually measured by the water?s resistance to electric current (in Ohm-cm). Deionized water quality depends on a variety of factors, including raw water composition, ion exchange resin types and quantities, and the number of resin tanks in the system.

Two-bed deionizers use separate tanks, one containing cation resin, and the other containing anion resin. A two bed weak base deionizer typically produces water in electrical resistance of about 50,000 Ohm-cm. A two bed strong base deionizer typically produces water with electrical resistance of about 200,000 Ohm-cm.

In a mixed bed deionizer, cation and anion resins are thoroughly mixed in a single tank. The mixed resins act like a series of alternating cation and anion exchange tanks to produce very high quality water. A mixed bed deionizer typically produces water with greater than 10,000,000 Ohm-cm resistance, which is equivalent to less than 0.05 mg/L of sodium chloride.

The resins need regeneration when they no longer produce the desired quality water. In the case of a two-bed ionizer, the cation tank is back-washed for 5 to 10 minutes, then washed with a 6 percent solution of hydrochloric acid. Then the anion tank is back-washed and washed with a 5 percent solution of sodium hydroxide. After rinsing residual chemicals from each tank, water flows through both tanks to drain until the water reaches the desired quality.

In a mixed-bed deionizer, the resins have to be separated before regeneration . After regeneration and rinsing, they have to be re-mixed, using air, before returning to service.

Although the process is fairly simple in concept, its application is complicated by variables in raw water composition, treated water quality needs, resin selection, chemical dosages and control system requirements.

The word "pure" water has different meanings where water is involved. Some people and some water departments claim water is "pure" when it is free of objectionable taste, odors, suspended matter, and colors, and it is safe to take internally. This kind of pure water may contain dissolved minerals in varying amounts, including the hardness minerals. To the medical profession, pure water will have all of the above characteristics, but it must be free of disease producing organisms. It must be sterile. To chemists, pure water is water that is low in dissolved mineral content, often extremely low, yet such water may or may not contain organic and other matter.

Many public water supplies are rendered ?pure? by filtration and/or chlorination. The medial profession relies upon distillation as the process for producing sterile water. Chemists may employ distillation, or they can use the deionization process.

How is specific resistance measured?

The less chemicals dissolved in water, the more it resist the passage of an electric current through it. This specific resistance is measurable in Ohms, the unit of electrical resistance. Natural waters have a specific resistance in the 1,000 to 5,000 ohms range. Deionized water may have a specific resistance indicated as 50,000ohms, or 100,000ohms, or 250,000ohms, or 500,00ohms, or 1,000,000ohms or higher. Ultra-pure water has a specific resistance in millions of ohms. Ohmmeters designed to read ohms in millions are calibrated in MEG0Ohms. MEG- Means "million". Six meg-ohms indicates six million ohms specific resistance. Theoretically, pure water has a specific resistance of 18,000,000ohms when measured at 25 °C. This value will change with a change in the temperature of the water, increasing as the temperature increases. Specific conductance is the ability of water to carry an electric current. The greater the mineral contents of the water, or solution, the higher its specific conductance.

What is a Micro-Mho?

When specific resistance is very high, or conversely, when specific conductance is very low, the unit of measurement is "MICROMHO". The prefix "micro" means "millionth". A micro-mho is equal to 1,000,000 divided by ohms. Deionized water with a conductance of 1.0 micro-mho has a specific resistance of 1,000,000 ohms. This is about of 1ppm of sodium chloride. Absolutely pure water has a conductance of 0.055 micro-mhos per centimeter at 25 °C (77 degrees Fahrenheit).

What is a conductivity meter?

A meter, either battery or electric current operated, which indicates the conductivity (specific conductance) in ppm of TDS as calcium carbonate or as sodium chloride. Some meters are calibrated to read in ohms and in meg-ohms. Meters work in conjunction with flow cells. Flow cells are positioned in the effluent stream and have two electrodes. Wires connect the cells to the meters.

What is a monitoring light?

A special indicating light operated with a flow cell that is positioned in the effluent piping. Some lights remain on and glow while the water quality remains within the desired range of purity. These lights go off when sufficient dissolved solids build up in the effluent to permit a small electrode current to bridge the gap between the electrodes of the cell. Some monitoring lights work the other way around. A special monitoring system contains two lights; A green light to indicate acceptable quality, a red light to show water quality is approaching the unacceptable level. A monitoring light may be coupled with a bell to create a bell alarm.

What is meant by cutoff point?

That point a which the quality of the deionized water is no longer desirable for use. This cutoff point varies according to the user's requirements. It can range from 25,000ohms (25K) up to 1,000,000ohms (1,000K), or even higher. Some monitoring systems have adjustable cutoff points; some have fixed points. In some cases, the monitoring system can be coupled with special valving. When the cutoff point is reached, the valves close, thus shutting off the flow of water to the deionizing equipment.

Is there a relationship between conductance and TDS?

Typically, the TDS is about 65% of the specific conductance. For highly mineralized waters and highly colored waters, the TDS is more than 65%. For water containing large amounts of acid, caustic soda or sodium chloride, the TDS is less than 65%.

What are some deionizer designs?

Deionizer designs fall into two general types.

1. Multi-bed

2. Mixed-bed

Some systems combine these two types.

What is a multi-bed design? Multi-bed means more than one bed of deionizer resin is required to make up a system. It may consist of one bed (in one tank) of cation resin followed in series by one bed (in one tank) of anion resin. Such an arrangement is a 2-bed system. Another arrangement is a 3-bed system, consisting of a tank of cation, plus a tank of weak-base anion, followed by a tank of strong-base anion resin. A still more elaborate system consists of a tank of cation plus a tank of weak-base anion, plus a tank of cation again, plus a tank of strong-base resin.

In some arrangements, a unit called a degasifier is placed in line after the cation tank. The purpose of the degasifier, or vacuum deaerator, is to remove carbon dioxide and/or oxygen from the water. This makes the water much less corrosive when used in steel equipment such as high-pressure boilers. Removal of the carbon dioxide also reduces the exchange load to a strong base resin in the system.

What is a mixed base design?

Mixed-bed means that two resins, cation and strong base anion (only), are carefully and thoroughly mixed in a certain ratio then added into a single tank. A typical ratio is 60/40 where the strong-base anion makes up the 60% of the total resin mix, the cation makes up the 40%. Mixed-bed deionizers are capable of producing water higher in chemical purity than is possible in multi-bed designs. A multi-bed system with strong-base anion resin can produce 100,000 to 500,0000ohm water. A single tank of mixed-bed resin can produce water with 1,000,000ohms resistance, or higher. Arranging two or more tanks of mixed-bed resin in series, can result in water purity reaching 18,000,000ohms. Some systems combine multi-bed and mixed-bed units. The former remove the bulk of the ions, the latter take out the remaining ions, thus giving larger volumes of high purity water than if mixed-bed alone were used. Mixed-bed units used in this manner are called "polishers".

What are the advantages of mixed-bed systems?

They take up less floor space, may cost less, and produce higher quality water. They also use less rinse water during regeneration. Regeneration is more complicated however, since the two resins must be separated physically within the same tank, and regenerated individually with different regenerants. In deionizer exchange tanks, mixed resins are removed from those tanks, and separated into individual regenerating tanks.

What determines the capacity of a deionizer?

The amount and type of anion resin determines capacity. Ratings are in grains of removed ions per cubic foot of resin. Rating depends on the type of anion resin, whether weak-base or strong-base, and the amount and kind of regenerant. Since weak-base resins exchange only chloride, nitrate and sulfate ions, they will have a higher capacity rating. Where deionizing systems using weak-base resins are involved, water analysis requires only determination of these "strong" anions.

Reverse Osmosis

Osmosis is a natural phenomenon in which a liquid. water in this case, passes through a semi-permeable membrane from a relatively dilute solution toward a more concentrated solution. This flow produces a measurable pressure, called osmotic pressure. If pressure is applied to the more concentrated solution, and if that pressure exceeds the osmotic pressure, water flows through the membrane from the more concentrated solution toward the dilute solution. This process called reverse osmosis, or RO, removes up to 98% of dissolved minerals, and virtually 100% of colloidal and suspected matter. RO produces high quality water at low cost compared to other purification processes.

The membrane must be physically strong to stand up to high osmotic pressure in the case of sea water, 2500 kg/m2. Most membranes are made of cellulose acetate or polymid composites cast into a thin film, either as a sheet or fine hollow fibers. The membrane is constructed into a cartridge called reverse osmosis module.

After filtration to remove suspected particles, incoming water is pressurized with a pump to 200-400 psi (1380-2760KpA) depending on the RO system model used. This exceeds the water's osmotic pressure. A portion of the water (permeate) diffuses through the membrane leaving dissolved salts and other contaminates behind with the remaining water where they are sent to drain as waste (concentrate).

TDS and pH

TDS and pH are indicators of water cleanliness, TDS and pH should be checked and recorded daily. As stated earlier, visual observation causes ineffective rinsing and water waste. The pH of water shifts up or down when it's used effectively. This shift depends on many factors. Following are the factors with the most impact:

Ideally, TDS and pH in rinse tanks should be the same as incoming water. Obviously, this isn't typical in actual production. In general for most situations, rinse-tank pH should be within +or- 1.5-2 points of incoming water pH, and TDS should be no more than twice the initial reading for rinse stages between active chemical tanks. Tests comparing TDS and pH values with adhesion, chemical use, and humidity and corrosion results are your only true way of knowing the answer.

You can figure that 1000Uhmo or more will be enough TDS to show up on your painted surface as a defect. Most water meters cleanliness is measured by using a TDS meter. Conductivity and TDS are related in that conductivity meter measures waters ability to allow an electrical current to flow through it.

Liquids, like high-purity water, which has few ions, are poor conductors. A conductivity measurement can estimate TDS levels in water. However, measuring the electrical conductivity provides only an estimate of the TDS levels in water because conductivity is not precisely proportion to the weight of an ion, and non-conductive substances cannot be measured by electrical loss.

TDS are solids that have been dissolved in solution and exist in ionic and non-ionic form. An example of this would be Isopropyl Alcohol. Even though it has a very high purity level, if you attempted to measure the conductivity you would get a zero reading. It is known that Deionized or DI water rinsing leaves rinsed substrates in a slightly acidic state.

The reason for this is as follows: In the DI exchange process, the resins take out everything including carbon dioxide, carbonic acid/CO2 and alkalinity. This is what leaves water very pure. Once this water is released to the atmosphere from the exchange process, it will start to absorb carbon dioxide or CO2. It will continue to absorb CO2 and without the alkalinity to buffer it, the pH will drop until a maximum saturation level has been reached resulting in a pH level always remaining at 5.0-7.0pH.

The reason for the slightly acidic state is that pure water does not re-absorb the alkalinity it processed out.

The reverse osmosis or R/O water process does not take out the carbon dioxide in its exchange process, but does take out alkalinity at a rate of 98% rejection rate.

So, if you had water content of 7.5pH to start, you would divide the alkalinity by the CO2. The result is a lower pH than you started with. Keep in mind that once you introduce the water to a substrate, the pH will rise because it will be taking on alkalinity.

Any slight change to the water will make your pH lower or rise because there is nothing to buffer it. If the water were left in a sealed pipe, it would stay at 5.5pH because no air can get to it.

Using R/O water as a rinse agent or tank fill can prove cost justified. It is important to keep your usage to a minimum yet sufficient to properly rinse your substrate. Effective rinsing is controlled by water cleanliness. You will use fewer chemicals in your bath and less neutralization will occur and your bath will ?live: longer between dumps. As for testing the water, you will probably be testing after equilibrium and if you are using a mixed bed or really clean DI water, you will need to get a resistivity meter. pH meters require the electrode to measure water that has conductivity in it. DI water if cleaned properly has very little conductivity in it. There is an ASTM method of adding some known salt "STANDARDS" to your water then testing it. Many companies try and test without these salts and end up with erroneous readings from their meters. They think their meter is not working when in fact, the meter just can't function with this water cleanliness.

Rinsing is an integral part of pretreatment for powder coat operations. It must function as a system with the other baths. Various rules of thumb can apply to all rinses. Some of these include:

Hope this helps you.....

Bob Utech
Powder Visions - Brooklyn Park, MN, USA


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