Nonisothermal Fusion Bonding in Semicrystalline Thermoplastics Christopher J. G. Plummer, Pierre-Etienne Bourban, Jean-Emile Zanetto, Gregory D. Smith, Jan-Anders E. Månson Laboratoire de Technologie des Composites et Polyme `res, Ecole Polytechnique Fe ´de ´rale de Lausanne, CH-1015, Switzerland Received 11 November 2001; accepted 16 May 2002 ABSTRACT: The development of the interfacial bond strength as a function of bonding conditions has been inves- tigated in two representative semicrystalline thermoplastics, isotactic polypropylene and polyamide 12. If one side of the interface is well above the melting point immediately before contact, more rapid effective bonding is obtained for a given estimated interface temperature than under isothermal con- ditions. This is discussed in terms of a simple two-parameter model for the critical strain energy release rate associated with crack propagation along the interface, which incorpo- rates the rate of establishment of intimate contact at the interface. The model provides a self-consistent phenomeno- logical description of the time and temperature dependence of the bonding kinetics in polyamide 12 joints, although questions remain regarding the detailed mechanisms of bonding. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1267–1276, 2003 Key words: polyamides; poly(propylene) (PP); fracture; toughness INTRODUCTION Industrial fusion bonding falls into three broad classes, depending on whether the heat required to provoke melting at the interface is (1) introduced from an external source, (2) generated in situ by friction or radiation, or (3) available from a prior forming oper- ation. 1–6 This last category is of particular relevance to integrated processing, currently under development in our laboratory for the production of multicompo- nent parts that combine the advantages of composites and neat resins. 7–9 The rational design of a sequence of forming operations will involve optimization of the total power consumption. Any heat (or mechanical work) introduced at a given stage of the process should, therefore, be exploited to the greatest possible extent during subsequent operations. If an injected component is to be fusion-bonded to a pre-existing component, for example, it may be advantageous to combine the two operations in a single overinjection step, rather than reheat the interface at a later stage. In the former case, joining occurs under nonisothermal conditions; that is, the surfaces to be joined are not at the same temperature immediately before contact. 7 This provides the motivation for this study, which focuses on the specific problem of nonisothermal fu- sion bonding, as previously defined, by comparing experimental results for two representative systems and introducing a new phenomenological model for data interpolation. The model is then used to derive simple processing maps for the optimization of over- injection within an integrated process. In the experi- mental investigation, both nonisothermal fusion bond- ing and fusion bonding under isothermal conditions have been considered. The latter is of less relevance to integrated processing, but it provides a useful refer- ence point when we consider nonisothermal bonding because isothermal bonding conditions are relatively well defined. 10 –13 Moreover, it is hoped that such a comparison may further our understanding of the factors that dominate the development of joint strength in each case and, therefore, prepare the ground for controlled experimental studies of the un- derlying mechanisms. In the past, fundamental studies of the thermally induced bonding of thermoplastics under well-de- fined conditions have tended to focus on amorphous systems and bonding temperatures just above the glass-transition temperature, T g . 14 –16 Such studies now often involve the characterization of the resis- tance to mode I crack propagation along the locus of the original interface with a linear elastic fracture me- chanics (LEFM)-based test. The results are then ex- pressed as a variation of the critical stress intensity factor, K c , or the critical strain energy release rate, G c , with the bonding time and temperature. It is usually Correspondence to: J.-A. E. Månson (jan-anders.manson@ epfl.ch). Contract grant sponsor: Swiss Progamme Prioritaire en Mate ´riaux. Journal of Applied Polymer Science, Vol. 87, 1267–1276 (2003) © 2002 Wiley Periodicals, Inc.