WATERBORNE DEVELOPMENTS AND THEIR EFFECTS ON PROCESSING

Waterborne coatings, for all their advancements, are still very different animals from the solvent-borne coatings they replace. Find out how, why, and how to cope here.

Research and development on water borne coatings began in the early 1950's, with the widespread adoption of latex emulsion based architectural coatings. Now, forty years later, the finishes have expanded in scope to include true solution coatings for use in the automotive industry, with finish quality that rivals and sometime exceeds the solvent-borne coatings that they are about to replace on a wholesale basis.

Abstract:
Waterborne coatings, for all their advancements, are still very different animals from the solvent-borne coatings they replace. Find out how, why, and how to cope here.

Research and development on water borne coatings began in the early 1950's, with the widespread adoption of latex emulsion based architectural coatings. Now, sixty years later, the finishes have expanded in scope to include true solution coatings for use in the automotive industry, with finish quality that rivals and sometime exceeds the solvent-borne coatings that they are about to replace on a wholesale basis.

Detail: [By: Mark Drukenbrod]

It was not until the advent of Rule 66 that development went careening ahead to provide coatings with less than 20% VOC'S. As a result, most of the advances have come about since 1977, when Clean Air Act Amendments were drafted. Interestingly enough, this flurry of development caused no formulation stone to be unturned, and a plethora of formulation systems were developed. These all fall under one of two general categories:

1. Emulsions (colloidal dispersions)
2. True solutions

The differences between the two are important for any discussion of manufacturing technique. Colloidal systems cured by coalescence of the resin particles. The high molecular weight of the system results in a paint film that has excellent pencil hardness characteristics. Due to formulation and the way the film forms, these coatings are the only air-dryers of the group. (That is, they air dry within an amount of time that is conductive to mass production.) One of the major advantages of the colloidal dispersion type of coatings is their ability to use high molecular weight polymers that will improve such qualities as color retention, water and chemical resistance. Until recently, these coatings were handicapped by lack of broad spectrum applicability, alternative polymer selection, and thick film disposition. True solutions come closest to resembling conventional solvent-borne coatings, except that they are formulated with water-miscible solvents as coalescing agents such as alcohols and glycol ethers, the majority of the vehicle system being water. These systems have been producing high quality, high gloss finishes on everything from refrigerators to Ferrari’s for about two years now, with durability approaching or exceeding solvent-borne coatings. Because of the way film forming and curing takes place in this type of coating, elevated temperatures curing are mandatory in most cases. Before we go on to take a look at the manufacturer of the coatings, let's take a look at the inherent problems that need to be formulated around in these coatings. For colloidal dispersions, most of the problems need to be formulated around in these coatings. For colloidal dispersions, most of the problems are in application. Spray guns tend to clog both because of particle size and drying characteristics. For this same reason, dip tanks tend to skin. They can exhibit flash rusting with ferrous substrates. They have, because of their formulation system, absolutely no freeze/thaw stability, and require all stainless steel manufacturing equipment. Also, they tend to offer much less gloss even under ideal application conditions due to film thickness and polymerization process. True solution systems take a long time to air dry, and are very critical of their spray environment and substrate preparation. They offer little in the way of long-term shelf stability, and tend to be processed and sprayed at lower solids levels than solvent-borne coatings.

The equipment manufacturer can suggest solutions for many of these problems, and the solutions tend to follow modern shear theory in a logical and predictable way. It must be admitted even by the most ardent "blue-sky" researcher, that anything can be done in the laboratory. But only the processing ideas that are readily and practically translated to mass manufacturing processes will ever succeed. These solutions must offer a good balance of economy, technology, and finish quality. Coatings manufacturers, due to their very competitive market, must maximize the following finished product characteristics:

1. Ease of manufacture
2. Cost of raw materials
3. Cost of manufacturing equipment
4. Quality of the applied coating
5. Ease of application
6. Labor cost component of goods produced

Different companies weigh these characteristics differently, but suffice it to say that every company is in business to make money, and a slightly better mousetrap at double the price may not be attractive to the customer. This overview of the types of coatings and the market is important to what follows because it gives us some basis for weighing the processing changes that will be outlined later. In addressing the manufacturing technique of various coatings, it could be recognized that there are several stages that a coatings goes through that are common to all coatings types. First is the premix. In this step, the pigments are pre-dispersed in the polymer, solvent and vehicle system. This process should be accomplished at as high a solids level as possible to capitalize on the rheological advantages of higher viscosity materials under shear. The premix step is designed to break down agglomerates of pigments and get the discrete particles of pigment thoroughly wetted with the liquid phase of the premix. Of course, depending on the type and quality of the pigments used, this will be variably successful. The very worst case allowable is that the discrete particles be wetted and the air in the interstitial spaces of the pigment clusters be replaced by liquid phase. It is at this step that the coatings manufacturer can get into the worst trouble. A majority of this work is now being done with saw-tooth or "Cowles" type dissolvers. This did not pose much of a problem with solvent-borne coatings whose formulations were much less complex, and behaved in a much more predictable way. They tended to easily release entrapped air, because the solvents used had a much lower surface tension than water, which now makes up a majority of the formulation. A worker could begin by adding the liquid phase of the premix, dump in the required amount of pigment, and turn on the disperser and slowly (or not) bring the speed of the disperser up to the point where he saw a "rolling donut" in the tank. He often increased the speed of the mixer to the point where the spinning dispersion blade was uncovered, causing a characteristic sound called 'wildcatting" because of its similarity to a mountain lions roar. This was thought to be the point of peak operating efficiency; because at the time it was either not important or not known that this "wildcatting" was the sound of air being minutely dispersed in the premix. After an hour or so, the premix was deemed to be sufficiently complete to be pumped to the milling device employed in the process.

In a modern waterborne coating, this is no longer the case. If this paper proposes one important point, it should be to spend more time and money on getting a good premix before the mill. In a waterborne, the pigment has much less affinity for the liquid phase in the premix, meaning that it tends to be harder to wet. In addition, most premixes tend to be considerably more shear thinning at a given solids level than a comparable solvent-borne. Overall, this means that the processor must put more work into a material that by its very nature has a tendency to resist the addition of more kinetic energy. If he were to make the premix like a solvent-borne, he would stand the material on the tank wall, or at very least incorporate excessive air into the premix that is no longer eager to release it. This is not only a hindrance in the premix step; it becomes a major problem in the milling step. Particle size reduction using shear depends upon the setup of numerous continuous layers of material in intimate contact, each moving at a slightly different speed, but viscously coupled to each other. The pigment particles at the boundary of these shear layers are then pulled apart due to hydraulic forces between the layers. Air introduced into the material causes discontinuities and weak points in the layers, acting much like the perforations in a roll of paper towels. The layers tend to tear and become inefficient transmitters of the very hydraulic forces meant to disperse the pigment particles. The solution to this problem is to provide an environment where very intense shear can be exerted on the premix material in a controlled manner without incorporation of air. The trusty saw-tooth disperser is obviously not the device of choice. The machine of choice should be a rotor/stator disperser. For most applications, a secondary pump or low speed blade will not be necessary, as the material should have good flow characteristics, depending upon tank dimensions. The rotor/stator device produces much more shear in a much more confined area than a saw-tooth disperser. The working end of the unit is made up of a multi-blade rotor rotating at high speeds in very close proximity to a durable but finely slotted stator. The clearances here are highly critical, and must be held to fractions of a millimeter for this application. Any axial deviation, even if it does not cause contact between the rotor and stator, will adversely affect the efficiency of the unit by causing an uneven flow characteristic through the head. The unit must also have a bearing in close proximity to the head to maintain clearances under operating stresses. The leading edges of the rotor should also be chamfered to enhance the head's pumping characteristics.

When compared to mill technology, this is inexpensive to accomplish, which makes it the most cost effective way to improve a waterborne process. Plan to spend 1/2 to 3/4 of what you would spend on the mill to achieve an optimally millable premix. This new piece of equipment will be doing much more work than the old disperser, so buy one that is durable and has plenty of power -- horsepower demand will be higher than a standard disperser, but not exorbitantly so. There are always alternatives, and this case is no exception. If the processor is unwilling to part with his saw-tooth disperser, he can operate it under vacuum, but this a distant second choice. It solves the problem of air entrainment, but does not otherwise change the premix material's behavior in the mixer. The material will still tend to move the tank wall at high disperser speeds, causing little work to be done to it. This causes longer processing times and less overall efficiency in the premix process. In a horizontal mill, grinding media reacts in an orderly way to the energy imparted to it by the milling disks. The media is accelerated due to viscous coupling by the material under process and forms a myriad of discrete layers, each moving a little slower as we move further away from the disk inputting energy. All of the media is moving in one direction in a more or less orderly fashion. Upon impingement upon the grinding shell wall, the media and product are forced to make a critical velocity change, using a great deal of potential energy. This forces recumbent flow back toward the milling shaft, forming the familiar "rolling donut" of laminar flow. Very little of the work that is done in a media mill is due to actual impact between beads of media. Remember that the media is viscously coupled to the product and since the energy density of a media mill is so homogenous, this causes the beads to be pretty much equally spaced in the grinding shell, in a sort of homeostatic energy equilibrium. Very few actual media collisions take place. The proof that collision is basically meaningless is provided by the fact that a charge of steel media of a given size performs just like a load of glass media of the same size. If impact milling were significant, as in a ball mill for instance, we would expect the much more massive steel media to perform efficiently. However, this is not the case.

There are several schools of thought on milling waterborne coatings, and each will be explored in order of efficiency. First, there is the "one-pass" school. These folks believe that the optimum way to mill a material is to run it through a mill with a very high energy density so that the final dispersion can be attained in one pass. With a good premix, this is possible with many pigments. All manufacturers have a certain leaning toward the one pass school, simply because of its unarguable efficiency advantage. There are certain problems here, especially with hard to grind pigments like quinacridone reds. The chemistry of the vehicle and binder systems of water-bornes is considerably more temperature sensitive than comparable solvent-bornes. The resultant problem is that many high energy density mills tend to cook the formulation in processing because they have insufficient wiped heat exchange area. The reason for this is fairly clear. These high energy density mills are comparatively small in volumetric capacity and mount anywhere from 10 to 15 horsepower per liter drive capacity. They are also meant to run at high media loading, meaning that this power is more efficiently coupled to the mix under process than in a conventional horizontal or vertical mill. The result is that the mix is sinking a considerably higher amount of kinetic energy, some of which will be turn into heat. The smaller volume means that the square heat transfer area is much smaller than in a conventional mill, hence the product in the mill exhibits a much higher delta (T) in processing. These mills look good on paper for waterborne applications; however, the real world issue of temperature control often causes them to fail in application. The next school of through is the recirculation or infinite dissolution school. The school believes that the way to a good dispersion is to pass from a cooled tank through the mill and back into the cooled tank until the proper grind is attained. The tank must contain a low speed sweep/pumper blade to keep the material in motion during processing, not so much to keep solids in suspension, but to keep the cooling surfaces of the tank swept with warm product for more efficient heat transfer. The mill involved is a detuned version of a standard horizontal mill. Why detuned? Again, the reason is temperature. In this milling system, the mix is recirculated for a long time and tends to lose viscosity and become harder to process as the temperature goes up. The less heat put into the mix per pass, the more slowly the temperature will rise. This system has been in favor throughout Europe for quite a while, but production takes considerable time, with a much larger labor component than a one pass process.

The next school of thought is the "multiplexers." This school utilizes mills with multiple milling heads -from two to four, each charged with progressively smaller media. These mills must be good coolers, but as the transfer of energy is a gradual process and energy density of the device is closer to standard, present (good) cooling technology is sufficient for the task. The reason for different size media in each milling head is that as the premix is pumped into the first shell, it can still have some fairly large agglomerates in it. These large agglomerates are most efficiently milled with larger media. This is mostly a function of the spaces between the media and the amount of shear developed in this space. At the exit of the first shell, the product has a much more uniform and smaller particle size. This material is then most efficiently processed with smaller media. The process is repeated through as many shells as is necessary to get the required grind. Most processors using this technique use two shells, although three and four shell mills are no longer all that uncommon in the industry. This discussion of milling theory begs the question: "What constitutes a good cooler," and moreover, how does a mill work and what design parameters can be modified to create the correct mill for a specific product?" Good cooling is the function of various variables. Among them are the L/D ratio of the shell, the input temperature of the water, the thermal constant of the shell material, and the square area of sink surface on the coolant side of the shell. A larger L/D ratio will allow more absolute transfer area per shell volume, that is, more area swept by warm product. In most cases, this also means more sink surface. The variable that exhibits the least amount of change is the thermal constant of the shell material. Stainless steels are known to be notoriously poor transfers of heat. (Note that most stainless steel cooking vessels are clad with a good heat transfer material such as aluminum or copper.) For this reason, the shell must be kept as thin as possible, while allowing for long wear. Most mill manufacturers have made this trade-off successfully by making the shell of work hardening stainless steel. This material is very abrasion resistant, and gets more so as it wears, allowing a thinner shell profile for better heat exchange without giving up longevity. The matter of heat transfer area is another area of trade-off. The best system involves a helical coil machined into the coolant side of the shell to increase transfer area. Providing the optimum setup for this is a simple matter of calculation, as providing additional cooling area in excess of the shell material's (T)q will yield no increase in cooling efficiency. Heat can only be removed after it sinks through the shell; the rate at which this happens is a property of the shell material and thickness, and not of the transfer area. What are the variable design parameters of the mill that permit customization? Most of these involve modifications of standard components for all manufacturers. One is the decreasing of the void volume of the shell by using larger, tapered spacers between the agitator disks on the main shaft. This serves two practical purposes. First, it actually increases the amount of energy transfer to the mill contents while decreasing the shell volume, allowing that additional energy to be coupled to a smaller volume of material. The reason for the increased energy coupling is that it eliminates the area of extremely low shear in close proximity to the shaft. Peripheral speeds are lowest here, and by increasing the diameter of the spacer and modifying it for good flow characteristics (making its profile a double cone rather than a simple cylinder,) energy transfer can be considerably increased. The next tweak is to increase the number of disks on the shaft by decreasing the length of the spacers. This must be done carefully, avoiding running out of main shaft horsepower. As a retrofit, this is easy to accomplish by noting the amperage draw of the main motor at your target flow rate. You can use this data to compute how many motor HP you are actually using. As a general rule, you should figure on 2-3 HP per extra disk, and allow for a 25% startup spike. As long as you are within the HP rating of the mill, you can be sure of increased efficiency by adding disks. You can get to a point of diminishing returns with this modification, however. As a standard mill is configured, there is a relative dead spot smaller. There is a point at which the dead spot is completely eliminated, and all of the media in the mill is moving with roughly the same amount of kinetic energy. This is the point of diminishing return. More energy can be added at this point, but it will do little actual work on the product in the mill. A logical extension of this point is that the first disk may increase horsepower demand by 2 HP, the second may only increase it by 0.5 HP, and so on, depending on how close to the point of diminishing returns you are getting. Experimental data for these modifications must be gathered under the exact same conditions with the exact same product. In certain cases, you may find yourself trading versatility for efficiency, especially if you plan to run more than one type of product in the mill. Since more work is being done in the mill, you might expect to run into heat problems with the product. By coincidence, there is also a dead spot at the shell wall between the convergences of the mill disk flow patterns in a standard mill. The increased agitation of the extra disks eliminates this dead spot, (actually causes better flow and heat transfer) and even though the energy dissipation in the mill is increased, temperature of the product will normally increase only slightly, or not at all. You can also change the type of disk in the mill to optimize efficiency in some cases. Pin disks are available that look like regular disks with 8 tungsten carbide pins (each about 1" long) let into the periphery of the disk. This is a very difficult assembly to make, as it involves several intricate machining and welding steps. Because of the cost of the disks, they are usually used in conjunction with a majority of standard disks, primarily to increase turbulence and energy transfer in a specific area of the mill.

Increasing the peripheral speed of the milling disks is another way to increase performance, although this comes at the cost of mill longevity. Higher peripheral speeds tend to cause more media contact with the wetted parts of the mill, causing increased wear. Additional tweaks would include increasing media loading percentage and decreasing the size of the media. How do these influence mill performance? For this explanation, we need to picture the media mill as a simple colloid mill with an infinitely long working surface or land. In a colloid mill, there is a rotor and stator of finite length whose gap is changed by physically adjusting the clearance between the two working parts. In a media mill, there are as many rotors and stators as there are working surfaces and beads. The media itself is the boundary surface against which shear is developed just like the stones in a colloid mill. These media beads are distributed evenly throughout the grinding cavity because of the homogenous energy density in the mill and the lack of gravity effects. If more media is charged in the grinding cavity, the beads redistribute, leaving smaller gaps between the beads. These smaller spaces are more numerous and tend to work much better on smaller particle sizes. This often increases efficiency in the mill, particularly in rheologically complex materials with relatively small beginning particle sizes, like waterborne coating formulations.

What happens if we use a similar charge of smaller media instead of merely increasing a charge of a given size? This is where the geometry of the media accounts for a more than proportional increase in efficiency. In explanation, let's visualize two different size media beads. Each bead is effective as a gap surface only over certain parts of its diameter, primarily because it is an arced surface, and the gap actually varies around the bead. In other words, there is a go/no go situation in the gap. Either there is enough of energy to overcome the adhesion forces of the agglomerates, or there is not. When two large beads are closely adjacent to each other, relatively little of the surface area of the bead is close enough to the adjacent bead to produce significant shear. For the sake of illustration, we will say that this is 1/16 of the media diameter. There are 5 other places on the bead where this takes place. Now imagine a much smaller bead and it's adjacent bead. Because of the smaller size of the media, proportionally more of the media's surface area is close enough to the adjacent bead to produce significant shear, in this case, perhaps 1/5 of the surface area is being used as a shear boundary surface. That means that nearly 100% of the external surface area, or 1/5 x 5 contact sites is being used to produce useable shear. Often, processors will lower media size and increase charge at the same time to yield a 7-8 times increase in milling efficiency. This is especially important in low viscosity systems like water-bornes as a way to increase shear and efficiency. Often in higher viscosity systems, the media will act as a filter bed or will cause the media to plug and stop product flow in the mill, causing the mill to cease operation due to excessively high product pressure. In terms of manufacturing, there are few surprises in either emulsion polymers or true solution water-bornes once the physical properties of the manufacturing equipment are accounted for and met. Latex emulsions have been available for many years, and recently, acrylic emulsions have become available which will enhance this type of coating's versatility. Emulsion polymer dispersions are gaining in popularity due to their improved chemistry that allows two stage curing or a core/shell effect. Many of these coatings are dry enough to handle in a very short time after spraying or dipping. They can even be sprayed electrostatically as long as neither the gun nor the substrates are at ground potential. As with all other colloidal dispersions, these are most stable at very small particle size. The polymer/emulsifier volume relationships must be held constant to achieve long term stability. Particle size can be a problem, as smaller polymer particles require more emulsifier, just as smaller oil/water emulsions need more emulsifier. The trick here is to do exactly the same amount of work to the mix in each batch, and meter the components carefully. Coincidentally, this is a perfect application for conversion to continuous inline manufacturing. Often, they can be made in a single pass on as inline homogenizer by metering the pigment dispersion, monomer, and other ingredients (anti-microbial, etc.) in one stream and the emulsifier, initiator, and water in the other. A useable product can generally be had with an inline unit that can generate shear in the range of 25,000 reciprocal seconds at reasonable flow rates. These materials can also be made in media mills, but they seldom require this much shear to manufacture effectively. In fact, a majority of latex emulsions are still made on high-shear dispersers. True solution waterborne coatings process much like solvent-bornes, except that the molecular-level electrical properties of water-bornes are much more complex. As a result of this, resin kick out during post-milling let down is far more likely in waterborne coatings than in solvent-borne. While the constituents of the formulation are very critical, the rates at which they are applied are also important. If the neutralizing base or the water vehicles are added too quickly, the resin tends to kick out. The symptoms are a crust of polymerized resin around the outside of the tank and a tank full of very thin pigment dispersion inside it. The reason for this is the relaxation of the polymer from its "ball" stage prematurely. Careful attention to letdown pH and temperature along with gradually more aggressive agitation are the cure for this problem, although no hard and fast rule can be given, because the actual cure will vary with the resin type, pH of the pigment dispersion and the makeup of the rest of the batch.

Chemical characteristics of the average waterborne are decidedly basic; while solvent-bornes are normally acid. Some pitting of very abrasion resistant stainless steels employing manganese as an alloying agent might be observed. This is seldom serious enough to require replacement of the part on the basis of decreased performance. The advent of the modern waterborne coating has caused the equipment manufacturer to rethink the way their product works and interfaces with the operator. Further advancements in terms of useful energy density and cooling in mills and controlled shear in pre-mixers are on the drawing boards all over the USA and Europe. In fact, due to the symbiotic and important relationship between the pre-mixer and the mill, many mill manufacturers are beginning to manufacture dispersion equipment of their own design to make sure that it is compatible with their milling equipment. As the state of water-bornes advances further, we will undoubtedly see more formulation changes, the adaptation of different resin systems, and other changes that may cause shifts in manufacturing equipment strategy. With the help of coatings manufacturers, equipment manufacturers will be equal to the task of providing equipment to make their new formulations.