PLURIDISCIPLINARY APPROACH OF STAINLESS-S TEEL POLYMER INTERFACIAL MECHANISMS

Stainless steel/polymer connections are currently on the increase indu strially for coatings, but also for sandwich structures where the poly mer is used between two sheets of stainless steel. This product posses ses good acoustic properties and is chiefly destined for applications in domestic electrical equipment (noise reduction in dish washers for example). Industrial manufacture is by on-line calendering. The aim of this study is to improve understanding of the reaction mechanisms occ urring at the steel/polymer interface in order to optimize the quality of the assemblies in terms of adhesion and durability under the condi tions which will subsequently be those of normal service conditions. T he interfacial interaction of a thermoplastic copolymer ethylene-malei c anhydride with stainless steel has been studied by infrared spectros copic techniques (FTIR) in order to improve the durability of steel/po lymer association, by modifying formulation of polymer or surface trea tment of stainless steel. In order to be successful, an analytical met hodology has been established. Interfacial chemical actions of maleic anhydride with stainless steel are simulated through thin layer of suc cinic anhydride (SA) on polished metal surface. FTIR spectroscopic (gr azing angle) permits a direct access of interphase for identifying new bounds between organic and metallic compounds. Interaction of succini c anhydride with the polar OH groups of the oxidized metal surface fol lowed a reactional mechanism which can be described by two steps: open ing of the anhydrid ring for reacting with adsorbed water and formatio n of acid group (-COOH); reaction of acid group with hydroxylic fracti on of metal surface and formation of carboxylate anion (-COO-) groups. This study will take the stability of the bonds formed between succin ic anhydride and stainless steel as a means of studying the longevity of the connections. Two different treatments will be applied: the firs t (200-degrees-C in the absence of oxygen) representing the calenderin g of the polymer between the stainless steel sheets and the second (40 -degrees-C in a damp atmosphere) representing ageing of the sandwich s tructure during use. FTIR observations carried out before and after he ating show complete disappearance of the acid functions and intensific ation of the carboxylate bands. Thus the latter are favoured by therma l excitation of the system, which should result in improved mechanical strength of stainless steel/polymer connections. The thin layers of S A on stainless steel are subjected to the action of water at 40-degree s-C. The acid forms present disappear very quickly, whereas the carbox ylate bonds remain stable for two hours (fig. 3). Then these products are transformed by hydrolysis, and all that remains is an absorption b and at 1682 cm-1 revealing the presence of hydrogen bonds, which thus show themselves to be more stable under hydrolysis than carboxylate bo nds proper. In a second time, steel/polymer association has been teste d after different conditions of elaboration and ageing, in order to un derstand the different mechanisms which occur inside the interphase st eel/polymer. Mechanical behaviour after heat treatment is improved, an d so, similar conclusions can be transposed to the structure after use , in domestic equipment. Modification of interactions between stainles s steel and polymer are caused, first by the chemical reactivity of an hydride functions, and second by the mobility of organic chains which reorganized inside the interphase. Analysis of surface failure show se veral correlations between mechanical behavior and chemical nature of residual polymer on metal substrate. Localization of failure depends o n conditions of ageing and can be explained by the <<theory of the min imization of interfacial energy>> between two materials of different s urface energy.