A study by Raman, near-infrared and dynamic-mechanical spectroscopies on the curing behaviour, molecular structure and viscoelastic properties of epoxy/anhydride networks
Introduction
Epoxy resins represent an important class of high-performance materials characterized by properties such as good mechanical and thermal behaviours, high resistance to solvents and corrosive agents, outstanding adhesion to various substrates, low shrinkage upon curing and easy processing under a wide range of conditions [1], [2]. These characteristics make them very attractive in a number of demanding, high-technology applications such as, for instance, the encapsulation of microcircuitry in the electronic industry, the development of specialized coatings for highly aggressive environments, the use as matrices for fiber composites in aerospace applications [1], [2], [3].
The more reactive and hence more widely employed epoxy hardeners are the amines (aliphatic or aromatic) for which the curing kinetics and mechanism are reasonably well established. Such deep understanding affords a close control of the final molecular structure of the networks by a suitable choice of curing conditions and composition of the reactive mixture [1], [2], [3]. In contrast, the information on the curing behaviour of multifunctional epoxies with carboxylic acid anhydrides is rather scarce, even though several of these systems have shown very interesting properties and considerable improvements with respect to their amine-cured counterparts. The epoxy/anhydride systems suffer the limitation of being less reactive, thus requiring higher curing temperatures and larger energy costs. This problem is generally alleviated by using suitable catalysts; Lewis bases such as tertiary amines or imidazoles are the most widely employed.
So far, the more comprehensive studies on the curing behaviour of epoxy/anhydride mixtures have been performed on non-catalysed systems [4], [5], [6], [7], [8]. These studies have evidenced that the main esterification reaction is accompanied by concurrent side reactions, among which etherification is favored. Less information is available for catalysed systems, for which several issues remain to be clarified [9], [12].
In the present investigation, an epoxy resin system of potential technological interest, namely tetraglycidyl-4,4′-diamino-diphenylmethane (TGDDM) cured with hexahydrophthalic anhydride (HHPA) with 2,4,6 tris-dimethylamino methylphenol (commercial name DMP-30) as an initiator, is investigated in detail. The tetrafunctional TGDDM monomer produces networks with very high cross-link density, which is reflected in Tg values exceeding 270 °C, and exceptional physico-mechanical properties: for this reason TGDDM cured with aromatic diamine hardeners such as 4,4′-diamino diphenylsulfone (DDS) are the preferred resin systems for use as matrices in high-performance fiber composites for aerospace applications. One major deficiency of these formulations is the absorption of relatively large amounts of water in high humidity environments, which brings about a general deterioration of properties [1], [2], [3]. The use of an anhydride hardener such as HHPA may result in a number of distinct advantages in comparison to DDS. Particularly relevant is the reduced toxicity of the hardener and the conspicuous decrease in water sorption it imparts to the formulations. Other relevant advantages are a smaller shrinkage and a lower reaction exothermicity [1], [2], [3].
One of the aims of the present investigation was to fully develop the potential of a number of molecular characterization tools that are believed to be the most appropriate for these complex networks. Particular attention was paid to choose complementary techniques which could be employed on the same sample, thus avoiding possible changes induced by the preparation procedure and/or the sample thickness or geometry. The second scope was to study the curing behaviour and the resulting viscoelastic properties of the investigated networks. Thus, the kinetics and mechanism of cure were analyzed at a temperature suitable for the system processing (e.g. 140 °C), in order to elucidate the role of side reactions, if any. The selected technique for this purpose was Raman spectroscopy, because it provides a number of key advantages in the present application, with respect to the more conventionally employed FTIR spectroscopy. These advantages are represented by the high quality Raman spectra produced by epoxy systems, the intrinsic sharpness of most bands, which improves significantly the resolution and hence the quantitative analysis, the possibility of monitoring large sample volumes rather than films few μm thick. A further important advantage compared to mid-IR is the possibility to use glass sample holders, not only because of the easier handling and cleaning of the cells, but especially because KBr, the window material for mid-IR, is reported not to be inert but to catalyse the curing of epoxies [7], [10]. Raman spectroscopy is very sensitive to polarizable bonds and less so to polar bonds. Therefore, groups such as hydroxyls are not readily detected, particularly when present in low concentration. For this reason the investigation of the post-curing process, which is very relevant for optimizing the curing schedule, especially in the case of non-stoichiometric formulations, has been carried out by means of NIR spectroscopy. This vibrational spectroscopy technique offers the advantages of being insensitive to fluorescence, capable of sampling specimens as thick as those used in the present study, and capable to provide accurate quantitation at trace levels [8]. The second part of the present contribution is devoted to the dynamic-mechanical analysis of the TGDDM/HHPA networks. The aim was to investigate the molecular structure of the network using a probe capable of providing information on segmental motions as well as on more extensive, cooperative transitions. The viscoelastic properties have been also analyzed in detail by using the WLF approach in order to investigate the effect of the stoichiometry, and hence of the cross-link density, on the molecular parameters which characterize the behaviour of these systems as viscoelastic solids.
Section snippets
Materials
The epoxy resin was a commercial grade of tetraglycidyl-4,4′-diaminodiphenyl-methane (TGDDM) supplied by Ciba-Geigy (Basel, Switzerland). The hardener was hexahydrophthalic anhydride (HHPA) obtained from Sigma–Aldrich Italy (Milan, Italy). 2,4,6 Tris-dimethylamino methylphenol (DMP-30) supplied by Sigma–Aldrich Italy was used as catalyst. All reagents were used as received with no additional purification.
The chemical structure of the resin components is shown below:
(a) TGDDM –
Curing mechanism
In the absence of catalysts the reaction between an epoxy monomer and cyclic anhydrides involves the hydroxyl groups, which act as initiators of the reaction. These groups are present on a fraction of the TGDDM molecules that underwent oligomerization on synthesis. The hydroxyl groups attack the anhydride forming a monoester with a free carboxyl group. This, in turn, reacts with the epoxide ring to yield a diester and a new secondary hydroxyl, which perpetuates the chain process.
In the presence
Concluding remarks
In the present contribution the curing behaviour of a TGDDM/HHPA formulation has been investigated by Raman spectroscopy. The quantitative accuracy and superior resolution of this spectroscopic technique allowed us to concurrently monitor the kinetic profiles of all reactive groups participating in the process, i.e. epoxy, anhydride and ester groups. Information has been gained about the mechanism and the side reactions (etherification) taking place at a curing temperature suitable for actual
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