Effects of humidity on polymer surfaces

The surface resistivity of plastic insulators in humid environments is a property of much practical concern, especially in electronics applications. Thus we find various methods of measurement and desired levels of resistivity under given conditions specified by the ASTM, the IPC and other agencies. Oddly enough, there is a dearth of published information about the actual behavior of such surfaces under various conditions of humidity. An old but comprehensive paper by Field¹ concludes that: 1) Surface resistivity of plastic dielectrics reaches an equilibrium value in times from 10 minutes to not more than one hour; 2) The equilibrium value is a strong exponential function of ambient humidity in the range 70% to 100% RH; 3) Regain of high resistance on subsequent exposure to 0% RH is generally as rapid as the initial drop at 100% RH or more so; and 4) Surface resistivity of plastics in equilibrium with a stated, high RH ambient is a material property, which permits these materials to be rated for order of merit in this respect.