energetic material, potassium perchlorate. (solid), loaded in the vessel. The 250 mL vessels were made of 304 stainless steel and polystyrene. The polystyrene ...
The 38th International Pyrotechnics Seminar, June 2012
Electrostatic Generation and Methods to Mitigate Electrostatic Discharge while using the ResonantAcoustic® Mixer P.A. Lucon, S.L. Coguill and Z.R. Martineau, Resodyn Corporation, Butte, Montana USA
ABSTRACT Mixing materials with ResonantAcoustic® Mixing (RAM) technology may cause electrostatic generation due to triboelectric effects without proper vessel configuration. These triboelectric effects are caused by interactions between the vessel wall and the mixing contents, as well as interactions within the materials being mixed. Experimental results indicate that an earth grounded, static dissipative mix container greatly mitigates charge build up on the vessel and in the materials being mixed. In addition, the acrylic hood used on the LabRAM® mixer has been observed to store charges during machine operation and during operator handling. Methods to minimize static accumulation on the hood are determined and reported.
Introduction ResonantAcoustic® Mixing (RAM) technology is currently being used to mix difficult to process materials that range from solid-solid materials to highly viscous materials, with, or without high solids loadings. The materials used in solid-solid mixing can vary greatly through various industries and for various applications. Anecdotal evidence suggests that triboelectric charges accumulate on the mixed material and mixing vessel when solid-solid materials are mixed. In this paper, static charge generation for solid-solid mixing is explored using a LabRAM® mixer. Static electricity is generated by any of or a combination of the following effects; triboelectric, piezoelectric, pyroelectric, as well as electrostatic induction.1 The most common method to generate static electricity is the triboelectric effect, which is driven by a materials affinity to lose or gain electrons. A material with weakly bonded electrons is more inclined to lose them. Whereas, other materials that tend to fill the outer electron shell have a propensity to gain electrons.1 The triboelectric effect is observed when materials come into contact with or slide past another material with a different affinity. Common material affinities are published on triboelecric tables, a sample of which can be found at http://www.trifield.com/content/tribo-electric-series/2. Materials at the top of the chart, tend to charge positive (loose electrons), while materials at the bottom of the chart are likely to become negatively charged (gain electrons). In Figure Acrylic Hood 3 1, triboelectric Headspace effects are caused by interactions between Materials being mixed the vessel wall and the mixing contents, Micromixing as well as interactions Vessel Wall within the materials Bulk mixing being mixed. Figure 1. ResonantAcoustic® Mixing Diagram.
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The 38th International Pyrotechnics Seminar, June 2012 The experiments performed and reported in this paper are: generation and measure of charge, means to mitigate charge, and application of mitigation of charge while mixing using the RAM technology. Experimental Generation and Measurement of Charge This experiment was to demonstrate that the vessel and material configurations could store a charge. The charge was generated using a hand-held Milty Zerostat charge generator. The electrostatic voltages were measured using an A-STAT-Check ESD hand held meter, by Bokar International,. Readings were taken at a distance of 50 mm from both the side of the mixing container and the top of the powder. Figure 2 displays the static generation and measurement configurations. Two vessel configurations were investigated; both configurations had a known static-prone energetic material, potassium perchlorate Figure 2. Static Generation and Test Configurations. (a) (solid), loaded in the vessel. The 250 mL Static Generation, (b) Measurement of Static on vessels were made of 304 stainless steel the Vessel, and (c) Measurement of Static in the and polystyrene. The polystyrene vessels M t i l are supplied with the LabRAM® mixers and the Table 1. Generation and Measurement of Charge Test Matrix. 304 stainless-steel vessel is Test Number Vessel Vessel Size Grounded Charge included with the heating 1 304 Stainless Steel 250 mL No Negative and cooling option for the 2 304 Stainless Steel 250 mL Yes Negative ® 3 Polystyrene 250 mL No Negative LabRAM . The vessels were 4 Polystyrene 250 mL Yes Negative tested in both grounded and 5 304 Stainless Steel 250 mL No Positive ungrounded configurations 6 304 Stainless Steel 250 mL Yes Positive as described above. The list 7 Polystyrene 250 mL No Positive 8 Polystyrene 250 mL Yes Positive of the tested parameters is displayed in Table 1. The charge was applied in 5-shot increments, from 0 to 20 shots. A shot was defined as one pull or release of the static generator trigger. After each 5-shot increment, the charge on the vessel and material was measured. After the 20-shot charge was measured, the configuration sat for 5 minutes and then the charges on the material and vessel were measured again. Both positive and negative charges were applied using the generator. In addition to the charges being generated on the mixing vessel, there have been reports of charges building up on acrylic hoods for the LabRAM®. Initial testing was performed using the charge generator under the following configurations: 1) A standard hood, 2) standard grounded hood, 3) standard
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The 38th International Pyrotechnics Seminar, June 2012 hood with a static dissipative film coating, and a 4) standard hood grounded with a static dissipative film coating.
Generation of Charge while Mixing Two mixing containers were chosen for these tests. The two vessel materials were 304 stainless steel and static dissipative polyvial (BAS) from Lacontainer. The static dissipative polyvial has an advertised decay rate of 5 kV to 0 kV in less than 2 seconds. The static dissipateive polyvial container has been reported to have been used on RAM systems for energetic mixing4. Potassium perchlorate was mixed in each container at 25 g, 50 g, and 75 g for 10 minutes. The vessels were tested with and without earth ground. The test configurations for the stainless-steel and polyvial vessels and the location of the ESD meter are displayed in Figure 3. Static Generation during Mixing (a) Stainless-Steel Vessel (b) Polyvial Vessel. Figure 3. Results Generation and Measurement of Charge The results of the experiments described above for the generation and measurement of charges are summarized in this section. Table 2 captures the charge results. The stainless steel container, even though it is conductive, stores a charge if it is not connected to earth ground. When the stainless steel container is tied to earth ground, the ground wire mitigates the vessel’s and mixing contents’ ability to store a built-up static charge. The static charge of the stainless-steel vessel and powder contents dissipates Table 2. Static Generation and Dissipative Results. Test Base reading 5 shots 10 shots 15 shots 20 shots 5 min wait Test Base reading 5 shots 10 shots 15 shots 20 shots 5 min wait
Stainless Steel Non-Grounded Negative Shots Positive Shots Powder Vessel Powder Vessel 0.5 kV 0.4 kV 0.6 kV 0.6 kV 1.3 kV 1.2 kV 0.0 kV 0.0 kV 2.7 kV 1.9 kV -0.2 kV -0.2 kV 2.4 kV 2.4 kV -0.6 kV -0.5 kV 3.1 kV 4.4 kV -1.0 kV -0.7 kV 2.3 kV 2.3 kV 0.0 kV -0.1 kV Polystyrene Non-Grounded Positive Shots Negative Shots Powder Vessel Powder Vessel 0.8 kV 0.4 kV 1.2 kV -0.5 kV 2.0 kV 1.1 kV -1.5 kV -0.7 kV 2.6 kV 1.3 kV -1.2 kV -0.8 kV 2.7 kV 1.6 kV -1.1 kV -0.5 kV 3.0 kV 1.6 kV -0.8 kV -0.5 kV 1.9 kV 0.4 kV 0.0 kV 0.0 kV
Stainless Steel Grounded Positive Shots Negative Shots Powder Vessel Powder Vessel -0.3 kV 0.1 kV 0.2 kV 0.1 kV 0.0 kV 0.3 kV -0.7 kV -0.3 kV 1.0 kV 0.4 kV -0.5 kV -0.6 kV 0.6 kV 0.0 kV -0.9 kV -0.4 kV 0.8 kV 0.4 kV -1.0 kV -0.4 kV 0.1 kV 0.0 kV -0.1 kV 0.1 kV Polystyrene Grounded Positive Shots Negative Shots Powder Vessel Powder Vessel 0.0 kV 0.0 kV 0.3 kV 0.3 kV 1.2 kV 1.0 kV -0.7 kV -0.3 kV 2.8 kV 1.8 kV -0.8 kV -0.3 kV 2.8 kV 1.8 kV -0.8 kV -0.3 kV 2.9 kV 1.8 kV -0.8 kV -0.3 kV 1.8 kV 0.2 kV -0.3 kV 0.0 kV
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The 38th International Pyrotechnics Seminar, June 2012 within 60 seconds when tied to ground. However, the polystyrene vessel and the powder stored a charge with and without vessel grounding. The powder and polystyrene vessel stored more negative charge (positive voltage) than positive charges. This is because polystyrene has a -70 affinity2 per the triboelectric table. Materials with a negative affinity are more prone to gain electrons than loose electrons, so it was expected that the polystyrene vessel would hold more negative charge. Tests were performed on the standard acrylic hood displayed in Figure 1, found in the introduction section. The cover tube, sitting on a wood table, was able to store 0.7 kV of charge. When the cover tube was grounded, its ability to store a charge compared to the ungrounded state did not change. This is because acrylic is non-conductive. By covering the cover-tube with an electrically conductive film, the cover tube’s ability to store a negative charge decreased, but its ability to store a positive charge increased. When the hood had a static dissipative film, and was also connected to earth ground, it was not able to store a measureable charge. The test results are displayed in Table 3. Table 3. Acrylic Hood Electrostatic Charge Test Results. Acrylic Hood NonGrounded
Test
Base reading 5 shots 10 shots 15 shots 20 shots 5 min wait
Positive Shots 0.0 kV 0.0 kV 0.1 kV 0.5 kV 0.7 kV 0.4 kV
Negative Shots 0.0 kV -0.5 kV -0.4 kV -0.2 kV -0.5 kV -0.3 kV
Acrylic Hood with Static Disipative Film and NonGrounded Positive Negative Shots Shots -0.1 kV 0.0 kV 0.0 kV -0.2 kV 0.0 kV -0.5 kV 0.1 kV -1.6 kV 0.1 kV -1.2 kV 0.5 kV -0.5 kV
Acrylic Hood Grounded Positive Shots 0.0 kV 0.3 kV 0.7 kV 0.8 kV 0.8 kV 0.5 kV
Negative Shots 0.0 kV -0.5 kV -0.3 kV -0.5 kV -0.6 kV -0.2 kV
Acrylic Hood with Static Disipative Film and Grounded Positive Negative Shots Shots -0.1 kV -0.1 kV -0.1 kV -0.1 kV -0.1 kV -0.1 kV -0.1 kV -0.1 kV -0.1 kV -0.1 kV -0.1 kV -0.1 kV
Generation of Charge while Mixing The generation of charge tests were performed using 100 grams of potassium perchlorate. Potassium perchlorate was chosen because it is highly used in fireworks, ammunition percussion caps, explosive primers, and is used in various propellants, flash compositions.5 The particle size of the material used was between 1 and 150 microns. The results of the charge generated on the vessel walls during mixing are summarized in Table 4. The only container that reported a measureable charge was the Polystyrene vessel. The remainder of the configurations tested; grounded and non-grounded stainless steel vessel as well as grounded and non-grounded blue polyvials, did not generate a measurable charge. The charge buildup inside the vessel was not measured. Table 4. Charge Build up on the Vessel Walls During Mixing on the RAM mixer at 25g, 50g, and 75 g of acceleration. Acceleration 25 g 50 g 75 g
Time 0 min 5 min 0 min 5 min 0 min 5 min
Polystyrene Non-Grounded 0 NT 0 NT 0 1.3
kV kV kV kV kV kV
Polystyrene Grounded 0 NT 0 NT 0 1.3
Stainless Steel Non-Grounded
kV kV kV kV kV kV
0 0 0 0 0 0
kV kV kV kV kV kV
Stainless Steel Grounded 0 0 0 0 0 0
kV kV kV kV kV kV
Blue Polyvial Non-Grounded 0 0 0 0 0 0
BluePolyvial Grounded
kV kV kV kV kV kV
NT ~ Not Tested
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0 0 0 0 0 0
kV kV kV kV kV kV
The 38th International Pyrotechnics Seminar, June 2012 Summary The experiments indicate that grounded conductive vessels mitigate static charges on the vessel. Also, a grounded conductive film on the acrylic hood mitigates any charge that would normally accumulate during machine operation and handling by an operator. References (1) (2) (3) (4) (5)
Static Electricity, Wikipedia, [Online]; Posted Online January 28, 2010. http://en.wikipedia.org/wiki/Static_electricity (accessed January 31, 2012) AlphaLab, Inc. http://www.trifield.com/content/tribo-electric-series/ (accessed February 8, 2012), contact (801) -487-9492 Lucon, P.; NAMF Mixing XXII Conference, June 2010. RAM Energetics Conference, July 14-15 Butte, Montana. 2011 Potassium perchlorate, Wikipedia, [Online]; Posted Online Febuary 5, 2012. http://en.wikipedia.org/wiki/Potassium_perchlorate (accessed March 26, 2012)
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