Aside from excellent resistant to chemicals, environmental stress cracking, low friction, weatherability, and low and high temperature operability, polytetrafluoroethylene (PTFE) has outstanding electrical properties. The PTFE molecule is symmetric, free of electrical charges, and resistant to polarization by magnetic or electric inducement. In contrast, polyvinylidene fluoride (eCF2eCH2e) contains permanent dipoles and can be further polarized.
Dissipation factor and dielectric constant values of PTFE vary little up to and beyond 10 MHz frequency. The value of dielectric constant is 2.1 essentially over the entire spectrum of frequency. PTFE dielectric constant and dissipation factors remain constant over a broad temperature range (40 to 240C). Dissipation factor of PTFE remains <0.0004 up to 100 MHz. It approaches a peak value at around 1 GHz. Applications of ePTFE include signal transmission (wire and cable), shielding in computers, telecommunications, and measurement cables. The low dissipation factor and dielectric constant of ePTFE make it an ideal insulation for many electrical/electronic products such as wire and cable.
In table, The porous structure allows signals to travel nearly at the speed of light with minimum loss or distortion plus thermal stability and mechanical flexibility. ePTFE also allows reduction of the overall interconnect size and weight.
Coaxial Cables:
Development of low noise cables (Fig.) for use in sensitive electrical signal transmission has been reported in a number of patents. The cable contained an insulation layer of ePTFE that was bonded using an adhesive such as tetrafluoroethylene copolymers like perfluoro ethylene hexafluoro ethylene copolymer (FEP) and perfluoro alkoxy polymer. The adhesive “fills” the membrane voids. The ePTFE layer was placed either directly or indirectly over a surrounding shield layer to maintain a fixed relative position between the insulation layer and the shield layer. The bonding process produced a tightly coherent interface between the insulation layer and the shield which is resistant to separation and movement in field application.
Because of elimination of the separation of the ePTFE insulation layer the “noise” in the form of triboelectric currents were significantly reduced in the cable while the beneficial characteristics of the ePTFE were preserved. Triboelectric effect is a type of contact electrification in which certain materials become electrically charged after they come into frictional contact with a different material. The efficiency of the cable to keep the noise done was sufficiently useful to eliminate the need for a low noise semiconductive layer to dissipate triboelectric currents in low noise cables. In addition to smaller size and lighter weight the cable has lower capacitance, improved flexibility, and reduced susceptibility to damage during use. Manufacturing time and cost are reduced thanks to ease of assembly and reduced material costs [8,9]. Typical applications of ePTFE-insulated cables include microwave coaxial assemblies which include test, aerospace, defense, telecommunication, and general purpose.
A hybrid flexible round cable with optimized electrical and mechanical characteristics for high performance in difficult applications has been commercialized. Rugged features, robust environmental protection, and high shielding capabilities have been developed for computer, industrial, and military applications with stringent performance circumstances. The benefits of such a cable include precise electrical properties, significantly lower capacitance, velocity propagation 85%, size reduction and weight for high-density applications, and high flex life in multiple axes.
Electromagnetic Interference Shielding Gasket:
An electromagnetic interference shielding (EMI) gasket is a conductive interface material that is used to connect an electrically conductive shield with a corresponding section of an electrical ground, such as a ground trace of a circuit board. EMI gaskets are required to shield circuit boards and other devices against interference. They are always, with minor exceptions, installed directly onto a conductive surface.
Optionally, the gaskets could be formed in place or precut to fit the circuit board. Manufacturing techniques for installing these gaskets include: (1) Dispensing a conductive paste or a conductive liquid material directly onto a conductive surface and curing the dispensed material in situ; (2) Die-cutting a conductive sheet material having an adhesive backer and then transferring, positioning, and adhering the dimensioned material directly to a conductive surface; (3) Mechanically fastening a conductive material to a conductive surface [17]. There are various methods of fabricating EMI materials; an example is described. Woven fabric of ePTFE fibers was coated on one surface with a dispersion of a thermoplastic fluoropolymer like FEP.
The FEP dispersion was applied by roll coating, followed by oven drying to remove the water yielding a 4% dry weight film [18]. The coated fabric was laminated to a porous ePTFE membrane by heating the porous ePTFE membrane on the surface of a metallic hot roll to a temperature high enough to melt the FEP dispersion particles. The FEP-coated side of the fabric was forced against the heated surface of the porous ePTFE membrane with a silicone rubber coated pinch roll. The porous ePTFE membrane had a thickness of approximately 25 mm, 95% porosity, and typical nodes and fibril's structure. The fabric constructed was laminated to an electrically conductive ePTFE containing electrically conductive carbon black particles using a layer of FEP film as adhesive assisted by heat and pressure using a hot roll and a pinch roll. The assembly obtained so far was laminated to an ePTFE membrane (76.2 mm, 90% porosity) using a lower melting thermoplastic polymer as adhesive.
A sample of the composite EMI shielding material was prepared and tested according to ASTM Method D4935. The shielding effectiveness (SE) was measured to be 26 to 30 dB. In another experiment the EMI shielding material was patterned, cut, and fashioned into a close-fitting cover for a shipboard radar housing. The cover was installed on the radar housing and field tested at sea over a period of about one week. The cover was repeatedly removed and reinstalled during the test period and electromagnetic radiation measurements were made inside the housing with and without the cover in place. Durability and EMI shielding performance of the structure during the test period were excellent and appeared to be unaffected by handling or weather conditions of the test. Average SE was determined to be 30 dB