TABLE OF CONTENTS Appropriate fillers have been used to produce adhesives with high thermal or electrical conductivity for specialized applications. The basic resins used include epoxies, urethanes, silicones, and polysulfones. Epoxies, however, are the most widely used resins. [21 ,22 ,23 ]
Synthetic resins are made electrically conductive by the addition of either metallic fillers or conductive carbons. The carbon can be either an amorphous carbon, such as acetylene black, or finely divided graphite. Usually finely divided silver flake is used in conductive epoxies and conductive coatings. Silver has the advantage of having moderately conductive salts and oxides so that slight oxidation or tarnishing can be tolerated. The resistivity techniques give much lower values than methods involving thin glue lines, such as ASTM D2739, where interfacial resistance plays an important role.[21]
Silver is preferable to gold as filler because it is less costly and has lower resistivity. Under conditions of high humidity and DC voltage, however, silver is reported to undergo electrolytic migration to the surface of the adhesive. Microspheres of silver-coated copper do not migrate, nor does gold. The highest silver loading possible is about 85% by weight. Silver loadings lower than about 65% by weight cause sharp drops in conductivity, but offer higher adhesive strengths. Carbon (graphite) gives fairly low conductivities.
Aside from silver and gold, other common metallic fillers include nickel, aluminum, and copper. Each of these metals presents particular compounding problems. Silver is often used in flake...
