The depth of moisture cure (z) has been measured for lap joints in alu
minium with a conventional sealant based on polydimethylsiloxane and o
ne based on polytrifluoropropylmethylsiloxane. The cured sealant acts
as a barrier to the permeation of water; any water which passes throug
h this barrier causes further cure which thickens the barrier. The dep
th of cure is given by z=(2PVpt)(1/2), where P is the permeability coe
fficient of water in cured sealant, V is the volume of sealant which r
eacts with 1 mole of water, p is the vapour pressure of water and t is
time.