Creep response of thixotropic ambient temperature cure adhesives measured by DMTA in static tension and shear Adlin S.M. Roseley a , Ester Rojo b,1 , Martin P. Ansell a,n , Dave Smedley c a Department of Mechanical Engineering, University of Bath, BA2 7AY, UK b Department of Chemical Engineering, University Complutense of Madrid, 28040 Madrid, Spain c Rotafix Ltd., Rotafix House, Abercraf, Swansea’ SA9 1UR, UK article info Article history: Accepted 26 May 2011 Available online 7 June 2011 Keywords: Adhesives for wood Epoxy Dynamic mechanical analysis Creep Thixotropic abstract The technology for bonding in rods into timber structures for repair, reinforcement and forming primary connections is now well established. An ambient temperature cure adhesive is required for bonding on site and for overhead application thixotropic (shear thinning) characteristics are essential. At the same time bonded-in components may experience service temperatures of 50 1C or more, especially in roof spaces. It is commonly supposed that an adhesive with a glass transition temperature (Tg) below the in-service temperature will suffer from potential creep unless it is tightly cross-linked. In this paper the creep properties of three epoxy-based, thixotropic adhesives are investigated, which are less heavily cross-linked and possess Tg values between 30 and 60 1C. The adhesives are subjected to a creep load in tension within a Dynamic Mechanical Thermal Analyser used in static mode with a step- wise increase in temperature and a range of stress levels. A unique laminated shear specimen has been developed comprising an adhesive layer sandwiched by two thin wood veneers so that the adhesive layer can be stressed in shear. The results demonstrate that in the temperature range between Tg and Tg þ15 1C the thixotropic adhesives creep to a limit, behaving as classic viscoelastic polymers and above Tg þ15 1C they behave like rubbers with no creep. At high stresses and temperatures the adhesives eventually fail by rupture of the adhesive bonds. In conclusion, thixotropic adhesives are seen to possess a unique combination of physical and chemical properties, which enable them to function above Tg under creep load. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction Adhesive bonding of wood has long been an accepted method for producing high integrity structures and components in the furniture, panel products, glulam and boat building industries. More recently adhesives have been used for the repair of timber structures [1] and for making timber connections with bonded-in rods [2]. Initially threaded steel rods were specified for these connections but pultruded reinforced plastics offer the advan- tages of low density and cost, compatibility with the elastic modulus of timber and the absence of surface preparation and corrosion problems [3]. Considerable attention has been paid to optimising the choice of adhesive [4]. It is generally the case that timber structural connections and repairs are made on site so that ambient curing of the adhesives is essential unlike the aerospace industry where bonded connections are made in an autoclave at elevated temperatures. Ambient cure adhesives have also been used for the reinforcement of timber structures [5], for example the Canadian bridge illustrated in Fig. 1. An essential feature of overhead reinforcement or repair is that when injecting adhesive into routed slots or drilled holes the adhesive should flow but after injection it should remain in place so that rods or plates can be inserted. A second attribute of these adhesives is an open time of up to an hour and a relatively quick cure time of about a week. For these reasons a range of thixotropic epoxy room temperature cure adhesives has been developed by the Rotafix Ltd. with shear-thinning and lubricant capability. The static and fatigue strength of these adhesives have been characterised [6–8] and their use in the repair of fractured timber beams has been demonstrated [9]. Nanostructure-property relationships have been investigated with respect to moisture absorption [10] and fracture toughness [11]. The adhesives are effective in wetting the surfaces of timber, steel and composite pultrusions and the void content of the cured adhesive is very low. Thixotropic epoxy adhesives possess glass transition tempera- tures (Tg) in the range 30–55 1C and the possibility of raising Tg Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ijadhadh International Journal of Adhesion & Adhesives 0143-7496/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2011.05.008 n Corresponding author. Tel.: þ44 0 1225 386432; fax: þ44 0 1225 386928. E-mail addresses: A.S.M.Roseley@bath.ac.uk (A.S.M. Roseley), erojorec@pdi.ucm.es (E. Rojo), M.P.Ansell@bath.ac.uk (M.P. Ansell), Daverotafix@aol.com (D. Smedley). 1 Fax: þ34 913944243. International Journal of Adhesion & Adhesives 31 (2011) 575–582