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Testing powder flow
F O C U S best of all, location of the classifier at the bottom of the mill. I still believe, however, that the action of recycling oversized particles back to the mill is not the best way to handle this material, since over loading a milling chamber is a retrograde step that can only increase the level of fines. From time to time I introduce case histories of new and novel application plants and conversions from liquid to powder. There are many examples of powder coating applied over an electrophoretic primer, but I had not previously encountered powder coatings applied over an autodeposition coating. I learn something new every day! Sid Harris
TECHNICAL Low gloss hybrid powder coatings A paper by John Schmidhauser of Sartomer discusses the use of a particular class of reactive agents that have been successfully used in low gloss powder coatings. Low molecular weight styrene maleic anhydride (SMA) resins are a family of anhydride and partial ester containing polymers that have been widely used for gloss reduction in thermoset epoxy powder coatings. Anhydride groups cure the epoxy resin, while the styrene groups ensure complete miscibility. A two-stage curing process of SMA-epoxy and epoxy-epoxy is believed to result in the fine surface textures observed in these low gloss epoxy powder coatings. Most formulators prefer polyester-epoxy hybrid powder coating systems in which the blend of epoxy resin with carboxyl terminated polyesters cure to give coatings with low cost and an excellent combination of resistance to yellowing and good 2
O N
POWDER
chemical resistance. It is, however, more complicated to achieve low gloss in hybrid systems than it is with the straight epoxy formulations, because of the necessity to balance the reactivities of the three resin components. If the reactivity of the three resins is similar then a high gloss film will be produced. Individual stages in the curing process include epoxy-epoxy reactions; anhydride-epoxy; polyester-epoxy; and anhydridepolyester reactions. There is another variable that has to be considered. This is the reactivity of the polyester resin, for they may contain a catalyst such as choline chloride or a phosphonium salt to accelerate the polyester-epoxy reaction. These catalysts are added during manufacture of the resin and are intended to speed the cure rate or lower the cure temperature of the standard high gloss hybrid powder coatings. Formulators have also found that these catalysts have a significant effect on the surface gloss of anhydride containing hybrid formulations. To examine these effects the reactions between each pair of resin components was subjected to DSC analysis. This is useful for assessing a wide variety of curing data including: the enthalpy of curing; time taken to cure at specific temperatures; temperature at which curing begins or exhibits a maximum in the curing exotherm at specific temperatures; and the extent of cure under certain conditions. These studies showed that all reactions are slow in the absence of an esterification catalyst, and the catalyst had the most pronounced effect on the reaction between the anhydride resins and the epoxy resin. This is the key to making low gloss formulations. In formulating low gloss hybrid powder coatings, the author suggests the following guidelines: The powder should be formulated with 5 to 10 percent
C O AT I N G S weight of an anhydride functional resin which is more effective in hybrid formulations than the partial ester grades. An excess of epoxy resin beyond that required to react with the polyester resin must be included. The coatings should be cured at a minimum temperature of 400°F. Ensure that an esterification catalyst is present by using a known catalyzed polyester, or by addition of tetrabutylphosphonium bromide at low concentrations of 0.25 to 0.5% by weight on the total formulation. Typical formulations are given in the article and the recommended SMA resin is SMA 3000. Article entitled “Achieving a Low-gloss Finish with Epoxy-polyester Hybrid Powders” by John Schmidhauser of Sartomer published in Powder Coating, Sep 2002, 51-55. Powder Coating is published 9 times each year by CSC Publishing Inc, 1155 Northland Drive, St. Paul, MN55120, USA
Testing powder flow In the broader view of the physical properties of powders an assessment of powder flow patterns is an important aspect of understanding some of the basic problems that can be associated with powder coatings manufacture, storage and application properties. It is estimated that powders account for 50% of materials used in industry, with an even higher proportion used in paint, coatings, and adhesives. Handling and processing powders, particulates and granules is fraught with problems because of their unpredictable and irregular flow behaviour. Powder handling tends to be problematic because powders exhibit properties similar to both solids and liquids, and since they are normally surrounded by air this can also affect their flow patterns. Problems occurring in feed hoppers include: ratholing; bridging; and flooding, and the factors affecting powder flow include the following powder DECEMBER 2002
F O C US variations: particle size; size distribution; particle density; particle shape; surface texture; cohesivity; particle interactions; attrition; water content and humidity; hardness; stiffness; thermal properties; critical relative humidity; compression properties; and ignition properties. External factors that can influence powder flow are: flow rate; extent of compaction; vibration; temperature; humidity; electrostatic charge; aeration; container surface; container shape; outlet diameter. and storage time. The initial methods for assessing powder flow are still widely used, despite their shortcomings, and recent technological advances. These include measuring the angle of repose by pouring the samples onto a horizontal surface and measuring the angle of the resulting pyramid; and measuring flow through a funnel. In the latter method, orifice sizes are changed to alter the flow out of the funnel, and it is usual to select the size through which the powder flows slowly and is reasonably constant. These techniques are influenced by the operator, they generally apply only to free flowing powders, and fall short of the standards required by modern businesses. The first and most common instrument for powder flow testing is the jenike shear cell. In this test the powder is loaded into the cell and then compressed by a defined weight. After compression, the operator measures the force needed to shear through the sample. Here again, the human involvement in the testing can affect the results, making them less reliable. In the past five years there has been intense development activity to provide truly accurate, repeatable and objective equipment to overcome the shortcomings of earlier test methods. A major factor behind these recent developments was the need for testing in environments which DECEMBER 2002
O N
POWDER
were as close as possible to actual processing conditions. Some of the most common powder handling problems arise when conveying from storage containers into production since the powder will generally have compacted to some extent and require more energy to instigate flow than is required with aerated samples. The latest powder flow analyzers are able to condition or aerate samples prior to testing, and realistic testing conditions are improved by controlling temperature and humidity within thermal cabinets. Modern powder testing equipment causes the sample to flow and users are able to define how the powder should flow. Movement of the sample is caused by a blade or rotor which passes through the powder at various settings that are defined by the user controlling vertical lift and rotation. Design of both the sample vessel and the blade is critical since the sample size is relatively small. Blemishes on surfaces, incorrect angles or edge faults would affect the flow of the sample and the test results. The blade can be set to perform many operations, including lifting or aeration to condition the powder and eliminate any loading variation. After compaction it can measure the level of particle cohesion, flow and recovery. It is also possible to examine compaction by rotation and downward blade movement when the sample being compacted will resist flow and movement. Slice compaction can be used to blend and mix samples. This procedure removes air from the sample and causes friction between the surfaces of granules and the smoothness of contact between the powder and the blade. These different movements can be carried out in any combination and sequence, and a simple test may involve conditioning, slice compaction and slice aeration, repeated a number of times,
C O AT I N G S followed by a blade shake to remove any adherent powder. A sophisticated device like the Stable Micro Systems powder flow analyzer can interpret and display the gathered data, as well as performing composite and statistical calculations. Powder flow analysis can be a valuable tool for comparing batch behaviour, and product quality before and after storage. Article entitled “Suppressing the Flow” by Jo Smewing of Stable Micro Systems published in Polymers Paint Colour Journal, Oct 2002, 192 (4457), 40,42-43
New milling techniques for powder coatings The ACM or Air Classifying Mill is well established in powder production lines although problems of excessive fines, temperature rise, and impact fusion of small particles have been associated with this milling technique. The trend to finer particle size distribution in powder production has aggravated these problems. To limit these deficiencies, Hosokawa Micron Powder Systems have developed a new milling technique which combines gravity-flow long gap milling and vertical axis air classification with an external circulation mechanism, that has been proved to give easy temperature control, low air flow rate, and fewer fines than the conventional ACM. In the new PCM mill it is possible to introduce raw materials up to one inch in dimensions and reduce the product to median sizes as low as 5-15 microns. The new design of the milling chamber has a long-gap milling rotor to reduce material which is input by a feed inlet at the top of the mill, or it can be air conveyed from the floor level. After dispersion within the mill housing, gravity flow draws the material through the long gap milling zone, which minimizes the possibility of retaining any fines or generating 3
TECHNICAL Low gloss hybrid powder coatings A paper by John Schmidhauser of Sartomer discusses the use of a particular class of reactive agents that have been successfully used in low gloss powder coatings. Low molecular weight styrene maleic anhydride (SMA) resins are a family of anhydride and partial ester containing polymers that have been widely used for gloss reduction in thermoset epoxy powder coatings. Anhydride groups cure the epoxy resin, while the styrene groups ensure complete miscibility. A two-stage curing process of SMA-epoxy and epoxy-epoxy is believed to result in the fine surface textures observed in these low gloss epoxy powder coatings. Most formulators prefer polyester-epoxy hybrid powder coating systems in which the blend of epoxy resin with carboxyl terminated polyesters cure to give coatings with low cost and an excellent combination of resistance to yellowing and good 2
O N
POWDER
chemical resistance. It is, however, more complicated to achieve low gloss in hybrid systems than it is with the straight epoxy formulations, because of the necessity to balance the reactivities of the three resin components. If the reactivity of the three resins is similar then a high gloss film will be produced. Individual stages in the curing process include epoxy-epoxy reactions; anhydride-epoxy; polyester-epoxy; and anhydridepolyester reactions. There is another variable that has to be considered. This is the reactivity of the polyester resin, for they may contain a catalyst such as choline chloride or a phosphonium salt to accelerate the polyester-epoxy reaction. These catalysts are added during manufacture of the resin and are intended to speed the cure rate or lower the cure temperature of the standard high gloss hybrid powder coatings. Formulators have also found that these catalysts have a significant effect on the surface gloss of anhydride containing hybrid formulations. To examine these effects the reactions between each pair of resin components was subjected to DSC analysis. This is useful for assessing a wide variety of curing data including: the enthalpy of curing; time taken to cure at specific temperatures; temperature at which curing begins or exhibits a maximum in the curing exotherm at specific temperatures; and the extent of cure under certain conditions. These studies showed that all reactions are slow in the absence of an esterification catalyst, and the catalyst had the most pronounced effect on the reaction between the anhydride resins and the epoxy resin. This is the key to making low gloss formulations. In formulating low gloss hybrid powder coatings, the author suggests the following guidelines: The powder should be formulated with 5 to 10 percent
C O AT I N G S weight of an anhydride functional resin which is more effective in hybrid formulations than the partial ester grades. An excess of epoxy resin beyond that required to react with the polyester resin must be included. The coatings should be cured at a minimum temperature of 400°F. Ensure that an esterification catalyst is present by using a known catalyzed polyester, or by addition of tetrabutylphosphonium bromide at low concentrations of 0.25 to 0.5% by weight on the total formulation. Typical formulations are given in the article and the recommended SMA resin is SMA 3000. Article entitled “Achieving a Low-gloss Finish with Epoxy-polyester Hybrid Powders” by John Schmidhauser of Sartomer published in Powder Coating, Sep 2002, 51-55. Powder Coating is published 9 times each year by CSC Publishing Inc, 1155 Northland Drive, St. Paul, MN55120, USA
Testing powder flow In the broader view of the physical properties of powders an assessment of powder flow patterns is an important aspect of understanding some of the basic problems that can be associated with powder coatings manufacture, storage and application properties. It is estimated that powders account for 50% of materials used in industry, with an even higher proportion used in paint, coatings, and adhesives. Handling and processing powders, particulates and granules is fraught with problems because of their unpredictable and irregular flow behaviour. Powder handling tends to be problematic because powders exhibit properties similar to both solids and liquids, and since they are normally surrounded by air this can also affect their flow patterns. Problems occurring in feed hoppers include: ratholing; bridging; and flooding, and the factors affecting powder flow include the following powder DECEMBER 2002
F O C US variations: particle size; size distribution; particle density; particle shape; surface texture; cohesivity; particle interactions; attrition; water content and humidity; hardness; stiffness; thermal properties; critical relative humidity; compression properties; and ignition properties. External factors that can influence powder flow are: flow rate; extent of compaction; vibration; temperature; humidity; electrostatic charge; aeration; container surface; container shape; outlet diameter. and storage time. The initial methods for assessing powder flow are still widely used, despite their shortcomings, and recent technological advances. These include measuring the angle of repose by pouring the samples onto a horizontal surface and measuring the angle of the resulting pyramid; and measuring flow through a funnel. In the latter method, orifice sizes are changed to alter the flow out of the funnel, and it is usual to select the size through which the powder flows slowly and is reasonably constant. These techniques are influenced by the operator, they generally apply only to free flowing powders, and fall short of the standards required by modern businesses. The first and most common instrument for powder flow testing is the jenike shear cell. In this test the powder is loaded into the cell and then compressed by a defined weight. After compression, the operator measures the force needed to shear through the sample. Here again, the human involvement in the testing can affect the results, making them less reliable. In the past five years there has been intense development activity to provide truly accurate, repeatable and objective equipment to overcome the shortcomings of earlier test methods. A major factor behind these recent developments was the need for testing in environments which DECEMBER 2002
O N
POWDER
were as close as possible to actual processing conditions. Some of the most common powder handling problems arise when conveying from storage containers into production since the powder will generally have compacted to some extent and require more energy to instigate flow than is required with aerated samples. The latest powder flow analyzers are able to condition or aerate samples prior to testing, and realistic testing conditions are improved by controlling temperature and humidity within thermal cabinets. Modern powder testing equipment causes the sample to flow and users are able to define how the powder should flow. Movement of the sample is caused by a blade or rotor which passes through the powder at various settings that are defined by the user controlling vertical lift and rotation. Design of both the sample vessel and the blade is critical since the sample size is relatively small. Blemishes on surfaces, incorrect angles or edge faults would affect the flow of the sample and the test results. The blade can be set to perform many operations, including lifting or aeration to condition the powder and eliminate any loading variation. After compaction it can measure the level of particle cohesion, flow and recovery. It is also possible to examine compaction by rotation and downward blade movement when the sample being compacted will resist flow and movement. Slice compaction can be used to blend and mix samples. This procedure removes air from the sample and causes friction between the surfaces of granules and the smoothness of contact between the powder and the blade. These different movements can be carried out in any combination and sequence, and a simple test may involve conditioning, slice compaction and slice aeration, repeated a number of times,
C O AT I N G S followed by a blade shake to remove any adherent powder. A sophisticated device like the Stable Micro Systems powder flow analyzer can interpret and display the gathered data, as well as performing composite and statistical calculations. Powder flow analysis can be a valuable tool for comparing batch behaviour, and product quality before and after storage. Article entitled “Suppressing the Flow” by Jo Smewing of Stable Micro Systems published in Polymers Paint Colour Journal, Oct 2002, 192 (4457), 40,42-43
New milling techniques for powder coatings The ACM or Air Classifying Mill is well established in powder production lines although problems of excessive fines, temperature rise, and impact fusion of small particles have been associated with this milling technique. The trend to finer particle size distribution in powder production has aggravated these problems. To limit these deficiencies, Hosokawa Micron Powder Systems have developed a new milling technique which combines gravity-flow long gap milling and vertical axis air classification with an external circulation mechanism, that has been proved to give easy temperature control, low air flow rate, and fewer fines than the conventional ACM. In the new PCM mill it is possible to introduce raw materials up to one inch in dimensions and reduce the product to median sizes as low as 5-15 microns. The new design of the milling chamber has a long-gap milling rotor to reduce material which is input by a feed inlet at the top of the mill, or it can be air conveyed from the floor level. After dispersion within the mill housing, gravity flow draws the material through the long gap milling zone, which minimizes the possibility of retaining any fines or generating 3