Key Attributes of Paint Filters
James E. Schmitz
Parker Hannifin Corporation
Process Filtration Division
Lebanon, IN 46052
Presented at FOCUS Conference 98
Filtration is a key part of the paint manufacturing process. Filtration is necessary to remove transient dirt, agglomerated pigments, gels ("fish eyes") and other contaminants which contribute to poor surface appearance properties. But a good filter must also not affect adhesion, color or other formulated properties. This paper outlines those properties of filters critical to obtaining high quality coatings. As a consumer or industrial user, you have certain expectations of paints and coatings. You want them to adhere to the substrate you are painting. Paints that peel are unacceptable. The paint should have good consistency, without gels or skins. It should provide a smooth covering without "fish eyes", craters or other surface irregularities. And you want the color to be the same in each can or drum you use. Paint Manufacturing Process
Paint manufacturers, of course, strive to produce paints with these key attributes. Paints are simply mixtures of resins, solvents and pigments (usually as dispersions). Other ingredients are added (formulated) to these major components to provide the characteristics needed for the specific application. The ingredients are weighed and mixed in a mixing reactor or tote. The entire process can take as little as 30 minutes to make the paint. Once mixed the paint is continuously circulated through filters to remove gels, grit, agglomerated pigments and other debris that would affect the surface characteristics of the paint. Samples of the paint are sent to an on-site laboratory where it must pass several quality control checks before it is released for packaging. Quality Control Checks
While there are many quality control checks for color, adhesion, solids level, etc., there are two that apply to surface characteristics. The first is a draw down test wherein the paint is spread on a glass plate with a wire wrapped rod. After drying the surface is examined for irregularities such as "fish eyes", craters, gels and particles of grit. Gloss and adhesion properties can also be checked. Surfaces are categorized as acceptable or unacceptable. The other test is a Hegman Grind test. A Hegman gauge is a rectangular block of stainless steel with a wide , inclined groove machined in the top surface. The gauge is graduated along its depth. Paint is placed at the deep end of the groove and spread toward the shallow end with a scraper bar. As contaminants larger than the space between the groove and scraper bar are encountered, scratches are formed along the groove surface. Each paint formulation has a specification for the position and number of scratches. If the paint fails either of these tests, circulation through the filters continues until subsequent tests are positive. Only then can the paint be packaged. Obviously the choice of the correct filters is critical to producing good, quality paints. Types of Paint Filters
The most common types of paint filters are bags and resin bonded cartridges. Filter bags are the choice for most industrial and commercial flat and textured paints. Resin bonded cartridges are preferred for enamels, glosses and automotive paints because they provide better particle removal efficiency. Sometimes filter bags will be used as prefilters for resin bonded cartridges. Bag filters may be mesh type or needled felts. Materials include polypropylene, polyester and nylon. Resin bonded cartridges are made from acrylic, polyester, fiberglass or cellulose fibers and are bonded with phenolic resin to provide rigidity and strength. These filters are usually depth type filtering from the outside to inside surfaces. Depth type cartridges have a graded density pore structure with larger pores on the exterior and smaller pores on the inside of the media. No matter what type of filter is used, it must have certain key properties: 1.) it must be silicone free, 2.) it must remove gels effectively, 3.) it should minimize the circulation time required before packaging, 4.) it should be capable of filtering a complete batch, 5.) it should not collapse or allow filter material to migrate into the paint and 6) it should not increase in efficiency during the filtration cycle so as to begin removing pigment particles. Silicone Free Paint Filters
Silicone is a release agent and even minute amounts in paint can adversely affect adhesion and appearance of paint. Silicone can cause paint to peel and can cause craters in otherwise smooth painted surfaces. Filters can be a source of silicone. Threads on filter bags or fiber additives and dyes on filter media may contain silicones. Even lubricants on the filter media production equipment and cleaning materials may contain silicone. Filter manufacturers must take precautions to eliminate sources of silicone. Paint manufacturers should require filter manufacturers to certify that their products are silicone free. Filtration of Gels
Gels are concentrated, highly crosslinked polymer networks caused by the presence of multi-functional monomers during polymerization of resins or uncontrolled crosslinking of linear polymers. They have high molecular weight and do not dissolve in the solvent for the parent polymer. Instead they become swollen, amorphous masses which can change shape under stress. Gels can be formed in the polymerization process due to differences in residence times, mixing rates and temperature within the reaction kettle. Because they deform under stress, they cause problems during filtration. With low pressure (stress) gels can flatten and cause blockage of filter surfaces. This reduces filter life. At higher pressure, gels can begin to penetrate into the depth of the filter and can elongate and work through the filter media. Figure 2 illustrates gel deformation with low and high stress. Depth cartridge filters are more effective in removing gels than surface filters like bags. Depth filters provide a thick, tortuous path increasing the likelihood of capture.. Gels could work through if the media deforms as the pressure increases. Therefore, rigid, resin bonded filters are preferred because they provide a stable fiber matrix. To increase the efficiency of gel removal lower than normal flow rates are recommended. This decreases the pressure drop through the media and increases its efficiency. Use of finer rated filters is also helpful to provide better gel removal capability. Decreasing Filtration Times
Paint manufacturers want to minimize the filter circulation time. Productivity increases if less time is spent filtering. Higher productivity means lower costs and higher profitability. Using the proper filter micron rating is the first step. Typically, metallic or textured paint require coarse filtration (125 - 150 mm) while flats and industrial paints can be filtered with 25 - 75 mm filters. Glosses and enamels should filtered with 2 - 10 mm cartridges. Secondly, filters should have sufficient life to handle a complete batch. Changing spent filters during a batch not only increases filter expense but also decreases productivity. Many filter manufacturers cut grooves into the cartridges to increase surface area and filter life. The cutting action creates saw dust which often attaches to the cartridge by static buildup. Unless adequately removed this saw dust can find its way into filtered paint. Some filters are made with short fibers or brittle glass fibers. These also can migrate through the cartridge into the paint. Often, failure to pass QC tests is a result of filter material that has come from the cartridge and migrated into the paint. Cartridges ideally should not have machined surfaces and should be made from long textile fibers. The Process Filtration Division of Parker Hannifin Corporation recently introduced its ProBond filter cartridge (Figure 3) which features two layer construction. The outer layer is a spiral ribbon with long, coarse fibers. It tends to trap larger particles and pigment agglomerates. An inner layer of long, fine fibers provides the required particle retention. The outer layer acts like a prefilter and tends to prevent the inner layer from premature plugging. Thus the cartridge provides longer service life without machined grooves on its surface. Media migration is minimized. Maintaining Consistent Color
Filters should remove large particles and agglomerates, not dispersed pigments. Paint filters should "classify", not "clarify". Classification is separation of particles as a function of size: large particles are captured and small particle, like pigments, are allowed to pass through the cartridge. Clarifying filters remove all (or most) particles without regard to size. As filters begin to plug by forming a cake, they tend to become more like a clarifying filter. Good classifying filters will have uniform fiber structures throughout the efficiency control layer and a controlled graded density construction. They will also be rigid to prevent compression of the filter media. Compression resulting from differential pressure increases filter density and changes filtration efficiency. Poor classifying filters include those made by vacuum forming. Here fine fibers in the liquid slurry tend to form at the ID rather than uniformly throughout the filter depth. As mentioned earlier, cartridges with low resin content tend to compress as the pressure drop increases. This increases efficiency and may contribute to pigment removal. Wound type filters tend to form filter cakes rapidly, thus changing efficiency during the filter cycle. Figure 4 shows filtration efficiency (as measured by turbidity removal) as a function of increased pressure drop for various filter types used in the paint industry. Each is rated at 25 microns by the manufacturer.
Note that the multi-layer cartridge (Parkers ProBond) maintains its initial efficiency well into its life. At 40 - 50 psid it begins to plug and the efficiency increases. Normal change out differential pressure is 50 psid.
The vacuum formed cartridge begins to increase in efficiency almost immediately. It is more of a clarifier than a classifier.
The low resin content filter act like a classifier until about 25 psid when the media begins to compress and the efficiency increases. The wound cartridge construction has lower initial efficiency than the others and shows an increase in efficiency when a cake begins to build at about 20 psid.Summary
The properties of paint filters necessary to produce quality paints include lack of silicone, good efficiency, long service life, ability to classify particles, cleanliness and rigidity. Of those filters discussed, the ProBond filter cartridge made by Process Filtration Division of Parker Hannifin Corporation best exhibits these properties. The ProBond cartridge is certified as silicone free, utilizes a unique, patented, two layer construction to increase filter life and strength. It is made from long, prime grade textile fibers to minimize migration. Manufacturing debris is minimized because there is no surface machining. The rigid, phenolic fiber matrix can withstand differential pressures up to 150 psid at ambient temperatures.
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