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Characterization of Novel Co-Anhydride cured Epoxy Resins

Rocks, Jens (2004) Characterization of Novel Co-Anhydride cured Epoxy Resins. PhD thesis, Queensland University of Technology.

Abstract

Epoxy resins are widely used as coatings, encapsulations, structural composites, castings, and adhesives in a number of electrical applications. Recently, novel uncatalyzed co-anhydride cured epoxy formulations that exhibit a high performance property profile, have been introduced. The objective of this thesis was to perform a comprehensive material characterization of these new resin/hardener combinations, which are potentially used as electrical insulation material in medium and high

voltage engineering.

The thermal cure behaviour of commercial tetraglycidyl-diamino-diphenylmethane (TGDDM) and a co-anhydride mixture consisting of maleic anhydride (MA) and hexahydrophthalic anhydride (HHPA) was extensively studied. Different analytical real-time methods, such as FT-Raman spectroscopy, differential scanning calorimetry, and chemo-rheological methods were applied to investigate the principal polymerization mechanism and the related curing kinetics. It was demonstrated that kinetic parameters that were based on isothermal measurements provided consistent and reliable results. On the other hand the general limitations of different dynamic

methods for kinetic parameter calculations were outlined and discussed. Temperature modulated differential scanning calorimetry provided a powerful technique to confirm a TGDDM/MA and TGDDM/HHPA sub-network structure of the co-anhydride cured epoxy. A generalized time-temperature-transformation-diagram was developed in order to predict the complex material transformations (e.g. gelation and vitrification) occurring during the entire isothermal curing process.

In the selected system, the mechanical deformation and fracture behaviour as a

function of temperature, strain rate, inorganic filler fraction, particle size, and filler/matrix-adhesion were thoroughly studied by using compression-, tension- and double torsion fracture-tests. The potential of hyperbranched polymers (HBPs) as low viscosity toughening modifiers for highly crosslinked anhydride-cured epoxy networks was experimentally evaluated. The effects of the HBP molecular structure, in particular the specific shell chemistry, on thermo-mechanical properties, final morphology, and blend concentration, were assessed. For the neat investigated epoxy-system the most efficient toughening modifier was obtained for a molecular

HBP-design that provided a pseudo-homogeneous blend morphology. Thus, by using suitable HBPs in a concentration of 20% w/w, the fracture toughness, expressed by the critical stress intensity factor (Klc) of 0.58 MPam(to the power of)0.5, was increased by over 50% to 0.88 MPam(to the power of)0.5. The corresponding Young's modulus and glass transition temperature were only affected to a limited extent by the addition of the HBP-additive. The toughest epoxy blend (critical stress intensity factor of about 1.6 MPam(to the power of)0.5) was achieved by the incorporation of 60% w/w inorganic silica particles. The application of hybrid concepts by utilizing synergistic toughening mechanisms (HBP and silica), revealed only moderate benefits within the investigated highly crosslinked materials.

As the examined epoxy/anhydride formulations are generally considered for high temperature applications, it was essential to determine their long-term thermooxidative ageing performance. The long-term thermo-oxidative ageing behaviour has been investigated by means of thermo-gravimetric analysis (TGA), dynamic

mechanical analysis (DMA) and vibrational infrared spectroscopy (FT-IR) methods with special emphasis on fundamental understanding of the ageing mechanism. Effects of the thermal ageing on the characteristic viscoelastic and flexural behaviour, weight-loss and oxidation susceptibility of the examined epoxy networks were assessed and discussed, thus providing an understanding of the principal material endurance properties.

The thermo-mechanical behaviour and the related structural changes with thermal ageing were examined by Cole-Cole plots in combination with a molecular theory previously developed by Perez. It was demonstrated that this new methodology provides a connection between conversion, glass transition temperature, and mechanical relaxation data and allows a fundamental molecular interpretation with respect to physical and chemical ageing phenomena.

A variety of thermo-gravimetric experiments were carried out in order to determine and model the specific weight-loss profile as a function of anhydride nature and ageing temperature. The influence of different inorganic fillers on the thermooxidative response was systematically studied. Different models were applied to extract meaningful kinetic parameters in order to describe the thermo-oxidative degradation and to facilitate an extrapolation of weight-loss data outside the experimental time- and temperature-scale.

FT-IR micro ATR-spectroscopy was used to identify, localize and quantify the

complex oxidation behaviour. A thermo-oxidative degradation mechanism, that

involved predominantly radical oxidation processes and C-N as well as C-O chain

scissions, was proposed to account for the experimental observations. Specific

oxidation-front profiles were constructed to describe the heterogeneous oxidation

processes.

Finally, the comprehensive material characterization of these novel co-anhydridecured

amino-glycidyl resins in terms of curing mechanism and kinetic, deformation

and fracture behaviour, and thermo-oxidative ageing performance, allows the assessment of the potential of these materials for demanding applications in electrical and electronic industries.

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ID Code: 15977
Item Type: QUT Thesis (PhD)
Supervisor: George, Graeme, Fredericks, Peter, & Greuter, Felix
Keywords: Epoxy Resins, Co-Anhydride, hexahydrophthalic, calorimetry, polymers
Divisions: Past > QUT Faculties & Divisions > Faculty of Science and Technology
Past > Schools > School of Physical & Chemical Sciences
Department: Faculty of Science
Institution: Queensland University of Technology
Copyright Owner: Copyright Jens Rocks
Deposited On: 03 Dec 2008 13:54
Last Modified: 29 Oct 2011 05:41

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