Triboelectric Charging Measurements
Static electricity has been known to damage and destroy semiconductor devices, unexpectedly initiate ordnance devices, ignite explosive atmospheres, cause undesirable consequences for workers 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. As a result, several tests methods were conceived to help evaluate safe materials in terms of ESD hazards.
The most common form of static electric charge generation is caused by triboelectricity. This is a result of charge deposition onto a surface after it comes in contact with another material and then separates. Testing a material's ability for charge generation has been successfully performed for more than 30 years at the NASA Kennedy Space Center. The newly formed EMPL is a direct descendant of this testing laboratory that developed the current standard used for the Space Shuttle and the International Space Station.
The current version of the triboelectric rubbing machine, the ESD Robot (above) is a fully automated robot capable of testing up to six materials in a variety of environments. This instrument is designed to measure both the electrostatic generating potential and the discharge time of the material under test. It is capable for testing films, clothing materials, space suits, solid foams, gloves, paints,coatings, solid surfaces up to 0.5 in. thick, and surface coatings such as anti-static coatings applied to thin solid samples. The robot operates in a laboratory test chamber (after proper material acclimation time) under nearly any environmental conditions:
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.
Normally, specimens are cut from the material to be
tested and then 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. After the polytetrafluoroethylene
(PTFE) rubbing wheel contacts the specimen for 10 seconds, the sample is placed
immediately in front of the electrometer and any charge generated along with
its decay rate are monitored using a data collection system.
The electrostatic acceptance criteria for the Kennedy Space Center is that materials must possess less than 350 volts within the first five seconds after rubbing. As one can see above a test of a certain cotton fabric that was thought could be used as a spacesuit material, failed the triboelectric test since the material maintained more than 700 volts after the first 5 seconds.
However materials that are well known insulators can pass triboelectric tests when a conductive underlayer is applied. Above, the decay of polyvinyl chloride with this conductive underlayer passes KSC triboelectric tests when rubbed with Teflon since the voltage is less than 350 volts after 5 seconds. The standard test environment for triboelectric testing is 75±5°F at 30% and 45% relative humidity. Special environmental conditions may be provided as needed depending on the customer's needs. The EMPL has the following environmental capabilities:
· Atmospheric pressure: 0.4 torr to 760 torr ( 0.533
mb to 1010 mb)
· Temperature: -190°F to 392°F (-123°C to 200°C)
· Humidity: 0.5% to 100%
· Various atmospheric gas constituents