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Additional physico-chemical information

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Endpoint:
additional physico-chemical information
Type of information:
experimental study
Adequacy of study:
supporting study
Study period:
Not specified
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: The study is a non-GLP and non-guideline study but provides useful information on the test material. The method utilised and the results obtained are well described in the report.

Data source

Reference
Reference Type:
study report
Title:
Unnamed
Year:
1999
Report Date:
1999

Materials and methods

Principles of method if other than guideline:
The objective of the testing was to provide data in assessing the propensity for producing propagating brush-type electrostatic discharges during transfer of the powder into metal totes coated with Belzona Supermetaglio 1341.
GLP compliance:
not specified
Endpoint investigated:
other: electrostatic hazard

Test material

Reference
Name:
Unnamed
Type:
Constituent

Results and discussion

Results:
The mass charge density (coulombs per kilogram, C/kg) of the test powder was calculated by dividing the total charge measured on the powder by the weight of powder transferred in each trial. The maximum mass charge density measured on the MILLAD 3988 powder after pouring was +1.5 x 10-8 C/kg under ambient humidity and was +3.7 x 10-8 C/kg under low humidity conditions.
The streaming current (amperes, A) for each trial was calculated by dividing the total charge generated (Coulombs) by the duration of each trial (seconds). The maximum streaming currents during pouring of MILLAD 3988 powder were +2.8 x 10-8 A and +7.5 x 10-8 A under ambient and low humidity conditions, respectively.
The maximum surface potentials measured on the powder pile were +5.8 and +5.3 kilovolts (kV) under ambient and low humidity conditions, respectively. The maximum surface potentials measured on the emptied Tyvek sacks were +2.5 and -5.4 kV under ambient and low humidity conditions, respectively.
The surface potential on the proposed Belzona Supermetaglio 1341 coating resulting from the charge on the MILLAD 3988 powder was estimated using Ohm's Law.
A resistance to ground of 2 x 1011 ohms was assumed for the coating, and was used with the streaming current for each trial to estimate the surface potential on the coating. The maximum estimated surface potentials for the coating were +5.6 kV and +15.0 kV under ambient and low humidity conditions, respectively.

Any other information on results incl. tables

One approach for reducing the probability of propagating brush discharges in the coated totes is to reduce the breakdown voltage of the coating to less than 4.0 kV by reducing the final thickness of the coating. Notably, the final thickness would need to be reduced by only 2 mils (to 18 mils) to bring the calculated breakdown voltage down to 3.7 kV (18 x 208 = 3744 volts or 3.7 kV) if the manufacturer's specification is correct (dielectric strength = 208 volts per mil). If reducing the coating thickness is chosen as the method for controlling propagating brush discharges, then the breakdown voltage of a sample of the coating as it is to be applied to the tote bins should be verified by testing. Alternatively, an antistatic or conductive coating or no coating may be used. Suitable antistatic or conductive coatings will have a volume resistivity of less than 1 x 1010Ω.m when applied.

Another approach for reducing the probability of propagating brush discharges is to reduce the resistivity of the powder to a value in the antistatic or conductive range (volume resistivity < 1 x 109Ω.m) and electrically ground the powder during filling and emptying The resistivity of the powder may be decreased by adding an antistatic additive or applying an antistatic coating to the powder. Notably, the resistivity of MILLAD 3988 was determined by testing to be 1.1 x 1014Ω.m. The volume resistivity of the treated MILLAD 3988 powder should be verified by testing. Grounding the powder may be accomplished by: (1) installing a grounded conductive plug or patch at the bottom of the tote; (2) using a grounded conductive bottom discharge valve; or (3) inserting a grounded conductive rod into the tote during filling or emptying. When properly implemented, this precaution would serve to dissipate electrostatic charge before hazardous potentials could develop.

Applicant's summary and conclusion

Conclusions:
If the coating (on the inside of the metal totes) becomes highly charged, an object at lower electrical potential approaching it could cause a propagating brush discharge when the electrostatic field intensity between the object and the coating exceeds the breakdown strength of air (300 kV/m). Alternatively, a propagating brush discharge from the charged coating could result when the electrical potential between the two faces of the coating exceeds its breakdown voltage (4.16 kV). As detailed in the results pouring one sack of MILLAD 3988 powder into another container can result in surface potentials as high as +5.3 to +5.8 kV on the powder pile; these values exceed the breakdown voltage of the coating. Further, surface potentials on the coating estimated from the powder streaming current also exceed its breakdown voltage (+5.6 to + 15.0 kV).
When the potential on the coating exceeds the breakdown voltage of the coating, a propagating discharge will occur through the coating. Propagating brush discharges may have an effective energy of as much as several Joules, well above the MIE of MILLAD 3988. If a propagating brush discharge occurs while a flammable dust cloud of MILLAD 3988 powder is present, ignition of the dust cloud could result. Additionally, propagating brush discharges can cause severe shock to personnel. Therefore, precautions must be taken to ensure that propagating brush discharges do not occur in the totes.
Executive summary:

Milliken Chemicals requested the testing described in this report in order to assess the propensity for producing propagating brush discharges during the transfer of the contents of 10-kg sacks of MILLAD 3988 powder into large metal totes. Suggestions were provided in the report of ways in which the probability of propagating brush discharges in the coated totes cold be reduced.