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RFI PROTECTION Sealed against RFI


Essex based EMC sealing and shielding company provides an overview of the materials and properties of its range of products available from its upgraded production facility.


E


MI shielding, O-ring and gasket specialist, Kemtron provides components for the defence, aerospace, information technology,


communications and electronics industries. The range includes conductive


elastomers and adhesives, form-in-place gaskets, oriented wires in silicone, woven and expanded wire in elastomers, knitted wire mesh and conductive fabric over foam gaskets To deliver this range, Kemtron has pioneered several proprietary manufacturing techniques, even designing some of its own manufacturing machines, with the help of a neighbouring company from the same industrial estate in Braintree. The range of products available is


very large, fulfilling the needs of the EMC protection industry for different materials and property characteristics.


SHIELDING MATERIALS EMC shielding materials are largely based around conductive elastomers – fully cured silicones or fluorosilicone loaded with highly conductive particles. The fillers ensure galvanic compatibility whilst providing low contact resistance between mating surfaces.


Silicone rubber (VMQ) is used because the end use requires a material that retains its elastomeric properties over a very wide range of temperatures and does not degrade due to the presence of oxygen and ozone. Silicone elastomers do have weaknesses in their properties and behaviour. In comparison with other elastomers their tensile, tear and abrasion properties are significantly poorer. However, they do not decay as the temperature is increased and above 150°C they become the better choice. The swelling and chemical resistance of silicones is comparable to those


2 /// EMC Testing Vol 1 No. 2


of chloroprene rubber, they are not affected by aliphatic oils; however they swell in napthenic and are attacked by hot aromatic oils. Silicones’ main weakness is to hydrolytic attack and decomposition, especially to steam between 120°C and 140°C; they are also susceptible to attack by acids and alkalis. Fluorosilicone (FVMQ) is used because it overcomes the chemical resistance and swelling weaknesses of silicone, whilst largely retaining the excellent high temperature properties. FVMQ however still suffers from attack by high temperature steam and hydrolysis by both acids and alkalis.


CONDUCTIVE FILLERS Kemtron manufactures conductive elastomers using four standard highly conductive fillers in both silicone and fluorosilicone variants. These are shown in Table 1. All have slightly different attributes.


SPECIAL MATERIALS Solid and sponge silicone gasket material populated with vertically oriented nickel alloy (Monel) or aluminium wires up to


140/sq cm provides excellent shielding with EMP survivability and will also provide an environmental seal. The material is available as sheet stock, strip or die cut and can be easily fabricated for large “picture frame” gaskets. Kemtron has developed new grades of


material using a very soft solid silicone with a reduced wire count of 100 wires per cm2. The advantage is that sheet widths of 225mm with a minimum thickness of 0.8mm can be manufactured. Material consistency is a great advantage


Table 1 – Principal conductive filler for EMC protection


Nickel Plated Graphite A high quality cost effective commercial material with increased use in the military markets. Easily extruded or moulded. SNG FR grade to UL94 V0.


Silver Plated Aluminium An excellent grade high performance material widely used for higher frequency applications in the commercial and military markets. Lighter in weight than some other materials.


Silver Plated Copper This material offers excellent RFI/EMI shielding performance across the frequency spectrum, but comes at a higher price and with increased weight.


Nickel This has largely been replaced by Nickel Graphite, but is still widely used in military and aerospace applications. The fluorosilicone version has better ageing properties than the silicone one. Overall, a good performer at lower frequencies but also heavier than some other materials.


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