Elsevier

Polymer

Volume 42, Issue 8, April 2001, Pages 3847-3858
Polymer

Toughening of a trifunctional epoxy system: Part VI. Structure property relationships of the thermoplastic toughened system

https://doi.org/10.1016/S0032-3861(00)00491-2Get rights and content

Abstract

This paper examines the effect of the addition of PSF upon the final properties and network structure of the TGAP/DDS system after cure and post-cure. It also compares the differences in the network structure and properties of the modified system between samples in which the epoxy resin and thermoplastic had been prereacted and those which had been simply mixed together. The thermal properties of the network structure were investigated using dynamic mechanical thermal analysis while the chemical structures were characterised using near infra-red spectroscopy. Physical properties such as water uptake, density and mechanical properties such as toughness, modulus, compressive strength and yield stress were measured.

Introduction

It is well known that epoxy resins cure to form highly crosslinked materials which are inherently brittle. This has limited their further proliferation into other applications which require more impact resistant or “tougher” materials. This drawback has prompted many studies devoted to increasing their fracture resistance without compromising their desirable attributes, such as their high glass transition temperature (Tg), high modulus and advantageous strength to weight ratios. The most common strategy employed has been to add a second “tough” phase, which is initially miscible in the epoxy resin but phase separates at some point during cure to form a thermoplastic rich phase and an epoxy rich phase. This produces a multiphase morphology which is able to initiate toughening mechanisms which can dissipate the energy of a propagating crack hence increasing toughness. The most common additives by far, to date have been carboxy terminated butadiene rubbers (CTBN), which are found to impart large increases in toughness particularly for the lower crosslinked epoxy resin systems. However, a disadvantage with rubber toughening agents is that they reduce desirable epoxy resin properties such as modulus and the glass transition temperature. For example, Sankaran and Chanda [1] found that the addition of 12% by weight of rubber caused the ultimate tensile strength and tensile modulus to decrease by 41 and 24%, respectively. They did, however, find that toughness of the resin system increased by 214%. For this reason, ductile engineering thermoplastics have been used as tougheners because they have high Tgs and moduli and thus do not compromise any of the desirable properties of epoxy resin systems. Early studies using thermoplastic toughening agents [2], [3], [4] were not particularly successful at increasing the fracture resistance of epoxy resins, but have prompted more studies which have investigated the effect on morphology, cure, thermoplastic endgroups and the chemical structure of the thermoplastic and epoxy resin. General criteria for thermoplastic toughening drawn from the literature are as follows:

  • 1.

    Thermoplastic backbone. This must have good thermal stability, should be soluble in the uncured epoxy but must phase separate during cure to form a multiphase morphology [3], [4].

  • 2.

    Morphology. Optimum toughness increase is achieved through attainment of a co-continuous or phase inverted morphology [5], [6], [7].

  • 3.

    Reactive endgroups. Their precise importance is unclear from the literature, but appears to depend upon the thermoplastic used [8], [9], [10], [11], [12], [13].

  • 4.

    Ductility of epoxy resin. Thermoplastics toughen highly crosslinked materials more effectively than they toughen materials with a lower crosslink density [14], [15].

  • 5.

    Molecular weight. Increasing thermoplastic molecular weight has been shown to increase toughness [16], [17], [18], the ease of processing being the limiting factor.

  • 6.

    Cure. The cure must be taken to completion to minimise unreacted functional groups, which introduce structural weaknesses [19], [20].

This paper is the sixth in a series of studies, which has studied the effect of thermoplastic addition upon the cure and structure/property relationships of an epoxy resin system. This work presents an investigation of the network structure and the thermal, mechanical and physical properties of the cured and post-cured thermoplastic modified system. The paper also investigates the effect of initial “prereaction” between the TGAP and PSF upon the aforementioned properties.

Section snippets

Epoxy resins and curing agent

The epoxy resin used in this study was tri-glycidyl p-amino phenol (TGAP; Ciba-Geigy), a tri-functional, low viscosity, amber liquid sold as Araldite MY0510. Prior to use, the resin was allowed to warm up to room temperature in sealed containers in order to prevent excessive moisture absorption. The curing agent or hardener used in all cases was 4,4′-diamino diphenyl sulphone (DDS; Ciba-Geigy), a pale pink powder sold as Araldite HT976. The thermoplastic modifier used in this study was a low

Near infra-red spectroscopy

A selection of NIR spectra is shown in Fig. 3, which shows clearly the epoxide peaks at 4522 cm−1 and the secondary amine peaks at 6577–6692 cm−1 that were used here to quantify the epoxide and secondary amine conversions.

The addition of PSF has previously been shown to cause the epoxide conversion of the cured TGAP/DDS network [24] to decrease. It is of great interest however, to find out how, both the epoxide and secondary amine conversions are affected by PSF addition and whether this leads to

Summary

This work has shown how PSF addition affects the chemical structure, mechanical and physical properties of the cured and post-cured TGAP/DDS network for the prereacted and non-prereacted samples.

  • 1.

    Near infra-red spectroscopy showed that during cure of the non-prereacted samples, all of the reaction occurred via epoxide/amine addition reactions, while during post-cure, most of the reaction takes place via etherification. For the prereacted samples, the amount of etherification that occurred

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