Aromatic tetra-glycidyl ether versus tetra-glycidyl amine epoxy networks: Influence of monomer structure and epoxide conversion
Graphical abstract
Introduction
Polymer resins for hot zones are increasingly in demand as fibre reinforced plastics and new applications for composites continue to emerge in the aerospace and space industries, the automotive [1] and oil & gas industries [2]. Some examples include, near engine components in aircraft, structural components in supersonic aircraft and space vehicles, high pressure pipelines and deep well drilling for hydraulic fracturing. The microencapsulation of printed circuit boards, where thermal, dimensional, and mechanical stability over wide temperatures ranges, particularly for short periods of time is another increasingly important application [3]. As demand for even higher service temperatures has grown, monomers and polymers based upon more inherently thermally stable structures have been developed, including cyanate esters (Tg ∼ 200–350 °C) [4], addition cured polyimides (Tg ∼ 250–370 °C) [5], advanced phenolic resins (Tg > 300 °C) [6,7], benzoxazines (Tg ∼ 320–370 °C) [8], and high-Tg thermoplastics (Tg ∼ 280 °C) [9]. Compared to epoxy resins, their high cost, often challenging processing requirements along with health and toxicity concerns have ensured that these materials remain mostly confined to niche applications. In contrast, epoxy resins have remained attractive polymer matrices and research dedicated to maximising their glass transition and service temperature continues. Indeed, epoxy-based composites currently make up about 80% of the global composite market [10].
The glass transition temperature of epoxy resins is typically well below 200 °C but can be up to 250 °C for multi-functional epoxy resins [11]. Despite the importance of glass transition temperature (Tg) in determining durability at elevated temperature; crosslink density, free volume, moisture ingress and residual stress, will all impact performance [12]. The tetraglycidyl diaminodiphenyl methane (TGDDM) and triglycidyl-p-amino phenol (TGAP) epoxy resins, for example, both have Tgs of the order of 250 °C but also contain glycidyl amine motifs which can enable side reactions and undergo internal cyclisation, to form mechanically inactive structures that prevent the network from achieving its ultimate properties [[13], [14], [15], [16]]. More recently, research into multifunctional highly aromatic glycidyl ether epoxy resins has shown promising enhanced thermal resistance due to the absence of this internal cyclisation. Liu et al. described the synthesis and characterisation of a tetrafunctional, nitrogen-free epoxy resin and showed that a blend of the monomer with diglycidyl ether of bisphenol A (DGEBA) had a Tg similar to TGDDM [17]. Further work by Xing et al. showed similar results using a nitrogen-free tetrafunctional epoxy when blended with up to 60 wt% DGEBA [18].
Another notable development in novel high-Tg epoxy resin is the synthesis of the monomer bis(2,7 glycidyl ether naphthalenediol) methane (NNE), an epoxy resin with a dimeric naphthalene designed to achieve a rigid backbone in the main polymer chain and a very high crosslink density. Ogura and Takahashi showed that when homo-polymerised using an imidazole catalyst, NNE exhibited a Tg above 320 °C [19] also suggesting that this was the highest Tg ever achieved by an epoxy resin. Additionally, they compared the dimeric naphthalene epoxies to a commercial epoxy cresyl novolac and noted consistently higher Tg at lower crosslink density, a characteristic they attributed to the rigid bis-naphthalene structure. To the authors knowledge it remains the highest published value of an epoxy network at the time of this paper. This epoxy resin is promoted for micro-electronics encapsulation applications where high heat and solder resistance is required [20]. While the very high Tg makes this resin attractive for high temperature applications, there is a lack of fundamental knowledge regarding its structure/property relationships, particularly when compared with the much more widely studied DGEBA, TGDDM, and TGAP-based networks. Recent work by Vukovic et al. utilised molecular dynamics simulations to study the structural origin of Tg and concluded that restricted segmental motions across cross-linked domains by the rigid dimeric naphthalene backbone of NNE, contributed to a markedly higher glass transition temperature compared with TGDDM [21]. More recently, our group contrasted the reaction mechanism, cure kinetics and network transformations of NNE to the more widely known TGDDM. When cured with DDS, the NNE initially cured more rapidly, undergoing gelation and vitrification sooner, and forming a more heterogeneous and topologically constrained network which ultimately limited the final degree of cure, despite having a significantly higher Tg than TGDDM [22]. An important difference between the glycidyl ether and glycidyl amine cure mechanism was that despite the inherent rigidity of the bis-naphthalene epoxy backbone, there was very little evidence of any side reactions such as homopolymerisation in the glassy state for the NNE compared to the TGDDM network.
Given the contrasting cure kinetics and phase transition behaviour of the different epoxy resins, this work builds upon this investigation to present a detailed structure/property relationship study of the glycidyl ether-based NNE networks cured with 4,4′-diaminodiphenyl sulphone (DDS) and compared against a glycidyl amine based TGDDM network also cured with DDS. A third formulation consisting of a 70/30 M blend of NNE and TGDDM respectively, cured with DDS was also investigated. The purpose of the work is to better understand the role of the highly aromatic naphthalene structure and the glycidyl ether functionality in determining the thermal properties including durability at elevated temperature, and mechanical properties of the resultant network. Although glycidyl amine and ether chemistries are generally well understood, in the context of thermally stable and highly aromatic network polymers, the impact of structural differences upon mechanical, thermal and physical properties have not previously been directly compared. To do this, the chemical structures of the networks were investigated using near infrared spectroscopy (NIR) and the thermal properties were explored using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMTA), thermomechanical analysis (TMA) and thermogravimetric analysis (TGA). Moreover, the physical properties such as density and water absorption were studied and fracture toughness and the flexural properties of the networks were also determined. The underlying motivation of the work is to investigate an emerging and understudied high-temperature epoxy resin with potential for applications in a wide range of demanding composite applications.
Section snippets
Materials
The tetrafunctional epoxy resin bis(2,7 glycidyl ether naphthalenediol) methane (EEW = 170 g/eq., Epiclon HP4710, NNE) was supplied by DIC Corporation, Japan. Tetraglycidyl diaminodiphenyl methane (EEW = 124 g/eq., Araldite M721, TGDDM) was obtained from Huntsman Corporation, USA. The aromatic amine hardener 4,4′-diaminodiphenyl sulphone (AEW = 62 g/eq., DDS) was purchased from TCI Chemicals, Japan. The chemical structures of the monomers are shown in Fig. 1.
Sample preparation
The monomers were placed in a
Network structure
The mechanical and thermal properties of crosslinked networks after cure are defined primarily by the polymer architecture and epoxide conversion. The NNE used here is based upon a tetraglycidyl ether and a bis naphthalene motif, which if fully cured should produce a highly crosslinked and thermally stable network. To explore the effect of the rigid structure and presence of glycidyl ether groups on the cure conversion and formation of the network, DSC analysis was performed. Fig. 2(a)
Conclusions
This paper presents for the first time, a detailed structure property relationship study of a DDS cured bis(2,7-dihydroxy-1-naphthalenediol) methane (NNE) epoxy resin to establishing the role of the rigid naphthalene motif and glycidyl ether conversion and compare it to a similarly cured glycidyl amine TGDDM and TGDDM/NNE blend, NTG. The rigid nature of the naphthalene, whilst imparting higher thermal stability and glass transition temperatures, also produced lower epoxide conversion, higher
Author statement
RV, SS conceived the ideas and designed the overall study. SS conducted most of the primary experimental activities while JG, CC, BG, provided guidance on experimental activities as necessary. The manuscript was written primarily by SS and RV, but reviewed critically by all authors.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Professor Russell J. Varley gratefully acknowledges support by the Australian Research Council (DP1801 00094) and the Office of Naval Research Global (N62909-18-1-2024). No conflicts of interest exist.
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