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Abstract:
Although it might look financially positive to take your color dispersion process in-house, make sure you see all the angles before making the leap.
Detail: [By: Mark Drukenbrod]
As you know, a peculiar series of concurrent business events have conspired to cause a period of sagging profits in the paint and ink industries. People are not willing to pay more for finished ink and paint, but the prices of the ingredients of the product have accelerated over the past couple of years. Since compensatory price increases were not a possibility, the bottom line of the company got squeezed. As a result of this, coatings manufacturing plants have economized in the areas of equipment (ouch!) and labor. They have also taken a look at the cost of products that they buy from other manufacturers. After all, they are manufacturers too, and the U-know-what flows down hill. Looming large in "the book of costs" is the cost of color dispersions. Dispersion is a color concentrate in which the pigment particles are pre-ground to their normal terminal size, and color has already been developed to a certain specified level. For providing this product, the dispersion manufacturer is paid handsomely, often up to 3 times materials and labor or greater (for particularly nasty stuff like Carbazole Violet).
People reason that this is an unreasonably high profit for something that they could do themselves from dry powder. Therefore, because of diving profit margins in the finished product, more and more paint and ink manufacturers are starting to make their own pigment dispersions, or are beginning to make a few of their largest volume dispersions, as a way to "shore up" the erosion on their bottom line. Now before we drag out the stake and start looking for our favorite dispersion manufacturer, let's see why he might actually be justified in charging the prices that he does for product. There are numerous great reasons why dispersions are so expensive. Every formulation parameter of dispersion must be the same, every time. The color development must be the same, every time...despite the condition of the pigment of which it is made. The actual formulation of the dispersion must be as general (read compatible) is possible, so that it can slot into the largest number of water or solvent borne coating formulations. Added to this, the dispersion manufacturer must have all of those neat dispersers and media mills which have a high capital cost. Dispersions must also be analyzed for color development, particle size, reflectance and refractance by very expensive little pieces of laboratory equipment. In short, the lot of a dispersion manufacturer is a tight-wire walk that would make the Flying Wallendas black out. For these risks, and because they can, they get the big bucks.
Now, having sufficiently dissuaded you from making your own dispersions, please allow this disclaimer in my defense. Not all dispersions are difficult to make. There are some that it makes sense to make yourself. Take for instance TiO2. This stuff is so easy to make, it is seldom even run through a mill. Most people make it on a standard HSD. Most organics are easy to do, especially Pthalo blues and greens. Barium reds are fairly easy, and Hansa yellows are not even beyond the scope of most coatings manufacturers. There are a couple of categories of dispersions that you will want to continue to buy out...trust me. Counted in this category are quinacridones, carbazole violets, and chrome yellows. As a mill peddler, these last three have taken more years off my life than being overweight and smoking combined. Granted, these dispersions are still pretty easy to make "once you know how"...it's just that getting to know how can drive you crazy, since each are heat, formulation AND energy dependant. Finding out how they inter-relate is why our teachers in high school were so intent and determined to teach us the scientific method. Most times concentrate formulation can only be correctly optimized by experimentation. Always keep in mind -- ONLY WORK ON ONE VARIABLE AT A TIME.
So, what kind of equipment are you going to need to start making your own dispersions? I like to suggest two different systems. One for making anything, including the difficult stuff...and one for the easy stuff, which will make almost everything that you will ever use.
Let's start with the no-holds barred system. First, you will need a disperser to make a premix. For most difficult pigments, you will want to make the premix at the highest possible solids level, since the object is to put as much energy into the product as possible. Since you will want conditions optimal to dispersion, I suggest a vacuum dual shaft disperser/mixer, with a normal high speed blade to do the deagglomeration, and a low speed pumping blade to move the material in and out of the high shear area directly surrounding the high speed blade. I have never had much luck with rotor/stator devices in this regard, although they impart more shears per unit area than a standard saw-tooth blade. The problem is getting material forced in and away from the rotor/stator head. This may be able to be done with a low speed turbine although it is not as easily done as with a standard disperser blade. Work your premix with minimum wetting agent (which defeats the inter-particle interference that you are trying to use to deagglomerate). Since you know what you will be using the dispersion for, you can add some resin solids, but as a general rule your dispersion should be, as much as possible, just pigment and vehicle. Work this at up to 70% solids and at viscosities up to 60,000 CPS in your vacuum multi-shaft disperser. As the pigment develops surface area, you might need to let down, so it is wise to hold back some vehicle in the beginning. You will pick up a lot of heat in this pre-dispersion process, and that's good since that will help keep your viscosity under control. When the viscosity and temperature cease to climb (usually about 30 minutes after your last addition), you've gotten about all your mixer has to give. Your product will look like colored cookie dough at this stage. The next step in this process is to let down to about 50% solids so you can get your crude, but well deagglomerated, dispersion through your media mill.
Your letdown for milling will depend on the mill that you are using. If you are using a mill with a screen of any kind, you may have to let down further than if you are using a mill with a gap separator. With a gap, shoot for a pre-mill viscosity of about 30 - 35,000 cps. For a screen mill, shoot at least 10,000 cps lower, or you won't be able to get the material out of the mill. During the milling process, the touchiest parameter is going to be pressure. Run the pressure right up to the red line to get the greatest efficiency from your mill. At this point the main drive motor will probably be drawing near maximum amperage and temperature will probably be fairly high. After the material has been judged to pass particle size specification, it must be let down to the level of solids loading that you normally use (most use dispersion at 20% solids). The object here is to add cold solvent (vehicle) to lower the viscosity and cool the material, along with adding any materials that you might require to stabilize the dispersion until you are going to plug it into a product. This is best done back on your mixer, using the low pumping blade only. This process makes you 99% of what you will require, and will make it the same every time, as long as you don't get creative with the batch card.
For an entry level system, I suggest starting with a standard high speed disperser. You will need to start working at about 55% solids, in order to limit viscosities to those pumpable by a HSD, (roughly 20,000 cps.) After your last powder addition, run the speed of your disperser up to about 5000 fpm peripheral on the blade. As the material heats, you will have to slow down the blade to keep the mixer from beating air into the premix. Keep the air out! It will kill your mill efficiency and limit the level of deagglomeration of your premix. Run the premix for about 30 minutes after your last addition. Ah yes! Additions. The ways these are made are particularly critical to this process. Do not dump complete 100# bags of pigment into the tank with one huge clunk! Add all powders gradually. Make sure you have a nice vortex at the shaft AND that you can't see any blade. Carefully and progressively pour the powder into the tank of vehicle. Speed the blade up as you add powder. Often, it will not be necessary to let the material down to mill it, but use the same guidelines as above. After milling is accomplished, let down to your use concentration and stabilize.
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