NEPTCO Tensile/Resistance Testing

Sam J. Speights
Chemist - NEPTCO, Inc.
EFranklin@chasecorp.com
Original publication date: Sept. 1998
Until now it has been difficult to determine the onset of loss in EMI/RFI shielding performance of shielding tapes when under strain. The most common industry practice has been to make a cable with the tape and test it. However, NEPTCO is developing an electro/mechanical lab test to measure shielding tapes’ shielding failure under strain. This test method measures the increase in electrical resistance of the tape during strain. It determines the portion of the resistance that comes from disruptions (cracks, tears) in the tape, (called Corrected Resistance - see Method & Terminology), its point of onset, and its growth. Tapes which show delayed onset of this portion of the resistance will have better shielding performance under strain.

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Click on the following links for charts showing samples’ Test Resistance (Rt), Corrected Resistance (Rcor), and Tensile Load versus % Elongation for a variety of PET/Al laminates.

Standard 48 gauge PET/0.00035 Al Laminate
Standard 92 gauge PET/0.00035 Al Laminate
Standard 0.00035 Al/92 gauge PET/0.00035 Al Laminate
Standard 92 gauge PET/1 mil Al Laminate

Results

All laminates tested showed negligible Corrected Resistance until about 15% elongation. This means that at these lower strains all laminates showed negligible electrical resistance evidence of breaks or tears. Presumably, if these tapes are never strained beyond these levels (whether during cable fabrication or use), there will be no loss of shielding performance due to such defects.

Between 15% and 20% strain, all standard laminates tested began showing signs of foil breaks. Presumably, shielding performance begins to drop steeply for these laminates at these strains. At higher strains all these laminates probably have drastically reduced shielding capabilities. To show this electrical property for all tapes we picked a 10% drop in corrected conductivity (corrected conductivity = 1/corrected resistance) to mark incipient tape disruption. We produced two graphs to show when this disruption began. Physical strength also suffers in standard laminates using 0.00035 foil, even if a lone statistic such as break elongation may appear superior. The standard 1 mil foil laminate is stronger initially but fails fairly early. These strength properties are probably the result of the breaking foils not adding their strength to the laminate.






The HIGH-DRAW laminates reached much higher strains before showing evidence of foil breaks. Unlike the standard adhesives, there is no clear range of strains in which we begin to see foil break initiation. Presumably, the HIGH-DRAW laminates maintain shielding performance better when experiencing higher strains. Laminates with 0.00035 Al also show higher physical strength at a given strain. The HIGH-DRAW 1 mil foil laminate shows higher toughness than its standard counterpart.
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Acknowledgements
Sincere thanks to Paul McClellan, NEPTCO Materials Scientist, for his assistance with these tests.

For data sheets and further information on NEPTCO's HIGH-DRAW products, click here.