Contact compatibility of organic vapours and their physical/chemical properties

The formation of carbon on the contact surfaces of commercial relays by thermal decomposition of organic vapours emanating from various organic materials may cause an undesirable increase of the contact resistance in the range of several Ohms. A two stage analysis is proposed, in order to find general rules for the influence of organic materials on contact reliability: The first step of investigation is to determine the identity and quantity of the gases emanated from interesting organic materials by chemical analysis. The second step is to investigate the influence of individual organic vapours and their combinations on contact reliability, using a specially developed test device. This test equipment exposes model switches to an atmosphere of synthetic air with a trace of one organic vapour. The concentration of this vapour is increased every 25.000 switching cycles in order to determine the so called critical concentration, when the contact resistance increases significantly due to carbonization. Critical concentrations of various organic vapours in combination with three different contact materials were measured. The relationship between these results and specific properties of the gases or their molecules was evaluated. First studies show the correlation between the contact compatibility on the one hand and a few parameters of the vapours or their molecules like volatility, the basic structure type of the molecule (e.g. aromatic or aliphatic, length of the carbon chain), and certain active groups on the other hand.