Elsevier

Polymer

Volume 41, Issue 2, January 2000, Pages 565-574
Polymer

The transport of water in a tetrafunctional epoxy resin by near-infrared Fourier transform spectroscopy

https://doi.org/10.1016/S0032-3861(99)00210-4Get rights and content

Abstract

An epoxy resin formulation composed of tetraglycidyl-4,4′-diamino diphenylmethane (TGDDM) and 4,4′diamino diphenyl sulfone (DDS) was investigated by Fourier transform near infrared (FT-NIR) spectroscopy and dynamic-mechanical analysis. Both techniques have demonstrated the essentially complete cure state of the resin with the adopted curing schedule. A novel feature of this work is the possibility of monitoring the transport of water into the resin by using of an FT-NIR spectrometer as a detector in place of the currently employed gravimetric detectors. The data gathered at different temperatures have revealed a good agreement with conventional gravimetric measurements, which verifies the reliability and accuracy of the experimental approach presented herein.

Introduction

The tetrafunctional epoxy resin tetraglycidyl-4,4′-diamino diphenylmethane (TGDDM) cured with the aromatic diamine 4,4′-diamino diphenyl sulfone (DDS), is one of the most widely employed matrices for the production of high performance fibre composites in the aircraft and spacecraft industries [1], [2]. The attractive features of this thermosetting resin are its low density combined with high tensile strength and modulus, and a very high Tg combined with good thermal and chemical resistance [3], [4].

It has, however, a very serious drawback for these applications, related to the absorption of large amounts of water, which brings about a deterioration in mechanical properties in hot moist environments. At equilibrium, a typical TGDDM/DDS cured system may absorb between 4.0 and 6.5 wt.% of water [5], [6], [7], depending on the stoichiometry of the formulation and on curing conditions. The absorbed water acts as a very efficient plasticizer, strongly reducing the Tg of the resin, typically by 20°C for every 1% of absorbed water [4], [6], [7].

Furthermore, if water absorption takes place at elevated temperatures and/or for prolonged time periods, a permanent damage of the structural network may ensue, which can give rise to the formation of microcracks and result into catastrophic failures [2], [3], [4], [7].

The mechanism of moisture absorption in the various types of epoxy has been widely investigated, but it is still not fully understood. One important issue is the state of aggregation of the water molecules within the bulk of the material. The penetrant population can be divided into molecules forming an ordinary polymer–diluent solution and those absorbed onto hydrophilic sites or trapped into the “excess” free volume frozen in the glass structure. Due to the multiplicity of states of the penetrant molecules within the matrix, the overall penetrant up-take cannot be considered as a reliable measure of the degree of plasticization.

Another relevant and controversial issue concerns the kind of molecular interactions between the water molecules and the epoxy network. Several spectroscopic studies, notably by solid-state NMR spectroscopy [8], [9], have addressed this point. Fourier transform infrared (FTIR) spectroscopy in the mid infrared range has also been employed [10], [11], but no definitive conclusions have been reached so far. This approach is exploited further in the present work.

Thus an experimental set-up has been developed, based on FTIR spectroscopy measurements, to monitor in situ and in real-time the transport process of water within a TGDDM resin cured with DDS. A near-infrared (NIR) wavenumber range between 8000–4000 cm−1 was chosen instead of the more widely employed mid infrared range (MIR, 4000–400 cm−1) for two reasons. The first is the intense and characteristic spectrum of water in the NIR range. The second reason is related to the fact that in the NIR interval are located the overtone and combination bands, whose intensity is about one order of magnitude lower than that of the corresponding fundamentals occurring in the MIR range. This makes it possible to use thick samples, i.e. up to several millimetres, without loosing the absorbance linearity with concentration of their spectra. Conversely, in the MIR interval the sample thickness cannot exceed 50 μm in order to remain within the absorbance range in which the Beer–Lambert law is valid. For such thin specimens the mass transport is very rapid and it would be difficult to study the relevant events even with a fast-scanning technique, such as FTIR spectroscopy.

Section snippets

Materials and curing schedule

The epoxy resin was a commercial grade of tetraglycidyl-4,4′-diamino diphenylmethane (TGDDM) supplied by Ciba Geigy (Basel, Switzerland) and the curing agent was 4,4′-diamino diphenylsulphone (DDS) from Aldrich (Milwaukee, WI, USA). The chemical formulae of both components is shown below:

The resin mixture was prepared by dissolving 30 g of DDS in 100 g of TGDDM at 130°C with vigorous mechanical stirring. After complete dissolution, the mixture was degassed under vacuum at the same temperature and

Characterization of the cured resin

In Fig. 1are reported the transmission FTIR spectra (NIR wavenumber range, 8000–4000 cm−1) for the uncured TGDDM resin (trace A), the uncured TGDDM/DDS formulation (trace B) and the same mixture after curing and post-curing (traces C and D, respectively).

Two characteristic peaks attributed to the oxirane ring can be identified in traces A and B at 6064 cm−1 and at 4524 cm−1[12], [13], [14], [15], [16], [17], [18]. The peak at higher frequency is due to the first overtone of the terminal CH2

Conclusions

A tetrafunctional epoxy resin, cured by an aromatic diamine hardener has been characterized with respect to the degree of cure by Fourier transform near infrared (FT-NIR) spectroscopy and dynamic-mechanical analysis. Both techniques indicated the essentially complete cure of the formulation in the conditions employed. The transport properties of the above resin have been investigated at different temperatures by a novel experimental approach which makes use of FT-NIR spectroscopy for detecting

Acknowledgements

Thanks are due to Mr. V. Di Liello for helping in the design and construction of the environmental chamber for FTIR desorption measurements and to Mr. A. Lahoz for technical assistance in the spectroscopic measurements. One of us (P.M.) acknowledges financial support by the National Research Council of Italy (CNR) “Short Term Mobility Program” 1996-1997, for his stay at Loughborough University to consolidate aspects of this work.

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