STABILITY OF ROOM TEMPERATURE CURE EPOXY ADHESIVES FOR TIMBER STRUCTURES UNDER CREEP LOADING IN TENSION AND SHEAR

摘要

Ambient temperature cure adhesives are commonly used for the repair and reinforcement of timber in buildings and may also be used where bonded-in steel or composite pultruded rods and plates are employed to make connections in timber structures. The thixotropic behaviour of such adhesives is an essential factor when overhead repairs or reinforcements arc being made in the field. Shear thinning characteristics are an important feature of such adhesives but glass transition temperatures (Tg) tend to be low and generally not much greater than room temperature. This paper addresses the issue of why connections bonded-in with thixotropic epoxy adhesives do not fail under constant creep loads in the field despite temperatures exceeding the material's Tg. A series of creep tests have been performed on adhesive samples under constant creep load in a dynamic mechanical thermal analyser (DMTA) whereby the temperature is increased in steps up to 80°C. The adhesive behaves as an elastic solid below Tg, as a viscoelastic solid just above Tg and as a low modulus, partially cross-linked, rubbery polymer at higher temperatures. Creep resistance is therefore a function of the cross-linked molecular structure of these adhesives. The assumption that the adhesives will creep uncontrollably above Tg is unfounded. The results of testing a miniature wood/adhesive/wood sandwich in shear in the DMTA are also reported, confirming the rubbery behaviour of the adhesive above Tg.