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remove the initial charge, but it will not alter it’s ability to conduct or dissipate
charge.
4.3 Effects of Humidity
Chemically treated antistatic materials are dependent on the relative humidity for
effectiveness. Both topically treated and internally mixed antistatic materials
draw moisture from the atmosphere to produce an electrostatically conductive
layer on the surface. Antistatic chemical concentrations may be such that at a
modest relative humidity of 50% the sample will exhibit excellent characteristics
with very fast decay times (less than 0.1 seconds), but may not exhibit any static
dissipative properties at 12% R.H.
Carbon or metal fiber loaded materials are not humidity dependent, but if the
loading is too light then the effects of the filler material will be dependent upon
humidity.
4.4 Insulative
Materials
An insulative material (one defined as having a surface resistivity greater than or
equal to 1x10
12
ohms/sq.) does not permit electrons to flow easily across the
surface. However, electrons can be removed or added triboelectrically to
produce a positive or negative charge on the material. When this occurs, the
sample is said to have an initial charge. This initial charge is detected by the
SAMPLE CHARGE
meter as soon as the sample is placed in the test
electrodes. If the entire sample is insulative, then when 5kV is applied the
sample will not conduct a charge and when the sample is grounded (depress
TEST
button) the sample will not bleed off the charge. When the 5kV is applied,
the
SAMPLE CHARGE
meter will read the algebraic sum of the initial sample
charge and the free air value described in Section 4.2.
If only a portion of the sample is insulative, then the
SAMPLE CHARGE
meter
will read an initial charge (not a calibrated value, however, because the "hot"
spot occupies only a portion of the field that is in view of the Electrostatic
Voltmeter sensor). When 5kV is applied, the
SAMPLE CHARGE
meter will read
the algebraic sum of the initial charge and the applied 5kV. When the
TEST
button is depressed, the sample will bleed off the applied charge and decay
down to the initial charge. This evaluation can only be done in the
MANUAL
mode.
Marginally static dissipative materials with very long decay times, and therefore,
very long charging times, can be evaluated by measuring the amount of charge
the sample accepts over a fixed period of time (e.g. 15 to 60 seconds, using an
external timer or watch). The accepted charge in this case is the charge
conducted across the sample after the 5kV has been applied (initial charge plus
free air value) to the value after the 15-60 second time period. The more charge
accepted within the established time period, the better the static dissipative
characteristics of the material.
