STANDARD TEST METHOD FOR EVALUATING

TRIBOELECTRIC CHARGE GENERATION AND DECAY

[TEST REPORT]{.underline}

Standard Test Method for Evaluating Triboelectric Charge Generation and
Decay

[ISSUED BY]{.underline}

National Aeronautics and Space Administration

Kennedy Space Center

Spaceport Engineering & Technology

Labs Division

PREPARED BY: \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

Rupert U. Lee/YA-F2-T

CONTRIBUTORS: R. H. Gompf/YA-C

1.  [SCOPE]{.underline}

This method describes the tribo-electric device used at Kennedy Space
Center (KSC) to test the electrostatic properties of materials,
including the preparation and conditioning of test samples, the
procedure used for testing, the data gathering system, and the procedure
used to verify measurement system accuracy. This method is especially
applicable for testing thin plastic films, foams, tapes, cloth fabrics,
solid surfaces up to 0.5 in. thick, and surface coatings such as paint
or anti-static coatings applied to thin solid samples.

1.  [SIGNIFICANCE AND USE]{.underline}

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1.  Attention has been given to the problem of static electricity
    because of its ability to damage or destroy certain semiconductor
    devices, unexpectedly initiate ordnance devices, ignite explosive
    atmospheres, and surprise workers doing critical jobs causing
    undesirable consequences and injuries to occur. These hazards
    associated with electrostatic discharge (ESD) are a continuing
    safety and financial concern to the scientific, aerospace, and
    industrial communities. Thin materials like plastic films, foams,
    and tapes are some of the materials most likely to develop damaging
    static charge buildup.

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2.  This test method may be used to evaluate the static electrical
    charge generated and the rate of discharge from materials under
    controlled environmental conditions.

3.  This test method is uniquely different from other test methods
    commonly used in industry. The uniqueness is that it addresses the
    combined effect of charge generation and dissipation. In contrast,
    surface resistivity measurement provides material information that
    is primarily related to the rate of charge dissipation. Decay rate
    measurement preceded by corona discharge also focuses on charge
    dissipation behavior. If necessary, these and other test methods can
    be employed in order to augment this test.

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1.  [SUMMARY OF METHOD]{.underline}

Specimens cut from the material to be tested are preconditioned to the
required test environment for 24 hours prior to testing. The test
specimen is then mounted in the sample holder and the system is
deionized bringing everything to zero potential. Then the
polytetrafluoroethylene (PTFE) rubbing wheel contacts the specimen for
10 seconds, and any charge generated and its decay rate are monitored
with a data collection system.

1.  [APPARATUS]{.underline}

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1.  [The Triboelectric Device]{.underline}

> Photographs of the complete test device and associated engineering
> drawings for its components are shown in Figures 1 through 12. It
> consists of a grounded aluminum frame with two cutouts in the front
> faceplate. The lower right opening exposes the Keithley 2501 static
> charge detector head. The upper left cutout is the pass-through for
> the rubbing wheel used to generate the triboelectric charge. The
> rubbing wheel is attached to a 1/8 HP electric drive motor with a
> speed control unit set at 200 rpm. A manual control lever is used to
> control the forward sliding motion of the motor/wheel assembly and
> provide gentle intimate contact control with the test specimen at the
> proper time. The rubbing force of the wheel against the sample is held
> constant during the test using a weight and pulley system. In this
> system a cord is attached to the motor assembly, run over a pulley,
> and a 3-pound weight is attached to the end of the cord. The test
> specimen is mounted in the grounded, aluminum sample holder in a taut
> condition. The sample holder is free to rotate about a fixed point
> facilitating rapid movement of the specimen from the rubbing wheel to
> the detector head.

2.  [Rubbing Wheels]{.underline}

> Each rubbing wheel has a diameter of 5 inches. The standard wheel used
> has a phenolic back, 1-inch thick foam cushion, and felt PTFE rubbing
> surface (Figure 11). The felt surface is used for most material
> testing configurations. A wheel with a solid PTFE rubbing surface with
> a convex surface radius of 12 inches is used on cloth fabrics, "wet"
> anti-static coatings, and test surfaces that would otherwise damage or
> contaminate the felt PTFE surface. Other special wheel configurations,
> such as a soft wool surface, can be made if needed.

1.  [Cleaning Rubbing Wheels]{.underline}

Some materials may contain or be coated with a substance that will come
off of the sample and be deposited on the rubbing wheel. This
contamination may then interfere with the accuracy of subsequent
testing. When this occurs, simply wipe the solid PTFE rubbing wheel with
a clean, lint-free cloth and reagent grade isopropanol. For the felt
PTFE wheel, fill a small tray with distilled water and place the felt
side down into the tray. Press down on the hard back of the wheel,
compressing the foam backing so the water can be absorbed. Compress the
wheel in this fashion a few times, then, with all of the water poured
out, compress the foam again to remove the excess water from the wheel.
Repeat the above step with reagent grade isopropanol. Place the wheel in
an oven at about 150°F for at least 2 hours until completely dry. Before
the wheels can be used again for testing, they must acclimate to the
test conditions for at least 24 hours.

> 4.3 [Data Collection System]{.underline}
>
> The generated charge on the test sample is measured with a capacitive
> static charge detector (Keithley 2501). This specific device provides
> 10,000 to 1 voltage reduction. The output of the detector is fed into
> an amplifier (Keithley 610C). Signal from the electrometer is then fed
> into an analog-to-digital data acquisition system, which is triggered
> by a 9-volt battery connected to a micro-switch on the motor/slide
> assembly. The triggering occurs when the rubbing wheel is retracted
> from the sample (ceasing charge generation) and also marks the zero
> time point on the data display page as the time charge generation on
> the test ends. Data can then be retrieved from the display in voltage
> vs. time. If desired, the data waveform may be stored on 3.5-inch disk
> for PC spreadsheet analysis or may be plotted immediately on a printer
> (HP 710C) connected to the data acquisition system.

1.  [PREPARATION OF TEST SPECIMENS]{.underline}

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1.  Cut three specimens, each about 8 inch square, from the material to
    be tested. Protective gloves should be worn to prevent contamination
    of the testing surface.

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2.  Condition the test specimens at the desired test environment
    (temperature, relative humidity, and atmospheric pressure) for a
    minimum of 24 hours before testing.

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1.  [TEST METHOD]{.underline}

> Once specimens are conditioned, the procedure for testing is as
> follows:

1.  Turn on Keithley 610C and data acquisition system and allow 30
    minute warm up.

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2.  Turn on 1/8 hp drill motor and verify speed of 200 rpm. Adjust speed
    if necessary.

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3.  Verify 3-pound weight is hanging free and moves freely with slide
    movement.

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4.  Verify data acquisition system and electrometer settings and change
    if needed.

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5.  Mount test sample in sample holder. This can be from one sample one
    side up to three samples two sides tested.

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6.  Extend rubbing wheel completely forward and turn on deionizer for at
    least 15 seconds to remove any residual static charge from the wheel
    and test sample.

7\. Retract rubbing wheel.

> 8\. Zero electrometer.
>
> 9\. Raise sample holder in front of rubbing wheel cutout.
>
> 10\. Move control levers to gently engage the rubbing wheels with the
> test sample and allow rubbing contact to continue for 10 seconds.
>
> 11\. While wheel is in contact with the sample, arm the data
> acquisition system and unground Keithley 610C.
>
> 12\. Quickly retract the rubbing wheel while simultaneously dropping
> the sample holder.
>
> 13\. At the end of the sample period, all peak voltages and the
> voltages measured at 0.5, 1.0, 2.0, 3.0, 4.0, and 5.0 seconds will be
> recorded and can be stored in the data acquisition system as well as a
> 3.5-inch disk.
>
> 14\. Repeat the test for all 3 samples. Both sides of the sample need
> to be tested, unless they are identical.

NOTE: Sequence of Steps 1 through 8 is not critical to test results.

1.  [CALIBRATION REQUIREMENTS]{.underline}

2.  Data acquisition system

> Because the electronics in the data gathering system are individually
> calibrated, an end-to-end calibration verification should be performed
> periodically to determine the reliability of the system as a
> integrated whole. This system verification should be performed any
> time a non-calibrated electronic component is replaced or when the
> triboelectric system is dismantled and moved. In order to perform this
> operation, a verification test plate must be fabricated as shown in
> Figure 14. This verification test plate consists of a thin, metal disc
> fastened to a sheet of insulating plastic. The plastic sheet is sized
> to prevent movement and electrical short of the disc while in the
> sample holder. A calibrated, high voltage dc power supply is also
> required.

1.  Turn on Keithley 610C and data acquisition system and allow 30
    minute warm up.

2.  Install test plate in sample holder with an insulated metal disk
    facing detector head. The size of the metal disk needs to be
    sufficiently larger than the detector plate of Keithley 2501. Make
    sure the metal disk is not in contact with any portion of the sample
    holder.

3.  Verify power supply is OFF.

4.  Clip grounded lead of supply to a metal portion of the sample
    holder.

5.  Clip the hot lead of the supply to the screw lug on the back of the
    test plate.

6.  Verify the settings on Keithley 610C and data acquisition system and
    make adjustments as necessary.

7.  Set the power supply to desired test voltage.

8.  Bring the rubbing wheel forward completely so that the data
    acquisition system can be triggered using the micro-switch circuit.

9.  Arm the data acquisition system and unground the electrometer.

10. Retract rubbing wheel and immediately turn on power supply.

11. When scope waveform is captured, turn off power supply and
    disconnect leads from test plate.

12. Verify that the steady state voltage on data acquisition system with
    the voltage output of the supply.

13. Adjust detector head spacing (forward or back) from test plate as
    necessary, repeating steps 3 through 12, until the system accurately
    agrees with the voltage from the power supply.

    2.  Other instruments

> A calibrated instrument with graduation/resolution of 0.5 ^o^C or
> better provides sufficient accuracy. The relative humidity sensor has
> preferably a sintered bronze tip, and is annually tested with the
> reference solutions defined in ASTM E104.

1.  [ELECTROSTATIC ACCEPTANCE CRITERIA]{.underline}

> Five (5) seconds after termination of tribocharging, the "worst case"
> voltage shall be 350 volts or less. Test results are valid only for
> the color pigment associated with the material being evaluated. The
> standard test environment shall be 75±5°F at 30% and 45% relative
> humidity. Special environmental conditions may be provided as needed.

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