Poly(ethylene-co-methacrylic acid) (EMAA) as an efficient healing agent for high performance epoxy networks using diglycidyl ether of bisphenol A (DGEBA)
Graphical abstract
Introduction
Intrinsic self-healing within polymers and polymer composites can be created either by incorporating reversible functional groups directly into a polymer structure or using a modifier with inherently reversible behavior [1], [2], [3], [4]. Typically activated by some external stimuli, these so called mendable resins have several advantages over more highly engineered alternatives, most notably their simplicity, repeatability and dormancy. Direct incorporation of reversible functionality into crosslinked polymer networks was pioneered by Wudl et al. [5] using thermo-reversible Diels–Alder chemistry to great effect. As a result, this [6], [7], [8] and other mechanisms including the use of hydrogen bonding [9], [10], free radical chemistries [11], [12], [13], metallic coordination [14], pi–pi stacking [15], and oxidation/reduction [16] to name a few, continue to drive this technology forward. Similarly mendable resins using additives with inherently reversible behavior, such as polymers that exhibit large viscosity changes with temperature [17], [18], [19], [20], miscible thermoplastics that migrate through nano-space [21], [22], thermoset/thermoplastic polymer matrices with controlled microstructures [23] and shape memory alloys [24] are also being actively investigated. Intrinsic healing therefore is an attractive self-healing technology contributing solutions to the challenge of service life extension and repairability for materials and components [25].
This study continues to explore the thermoplastic modifier poly(ethylene-co-methacrylic acid) (EMAA) as a healing agent in brittle epoxy polymer composites, but this time for high performance epoxy resins and not just for room temperature curing resins which has been the most common to date. Meure et al. [26], [27], first showed EMAA to be effective as an immiscible additive using reactive room temperature cure epoxy/amine resins and composites based on diglycidyl ether of bisphenol A (DGEBA) and triethylene tetramine (TETA). Studies have demonstrated that EMAA based healing is robust and efficient when used as different sized particles [28], [29], as fibres with varying diameter in a woven or non-woven fabric [17], [30], as z directional stitching of different densities [17], [18], [31], [32], [33], [34], [35] and finally as a blend with other epoxy resins [36]. Indeed, healing has consistently exhibited well over 100% recovery of fracture toughness, irrespective of form or composition when compared with the virgin polymer network or composite.
The healing mechanism has been shown to derive from a condensation reaction between hydroxyl groups from the epoxy/amine network formed during cure and the carboxylic acid groups of EMAA. This reaction however, requires catalysis by tertiary amine groups which are formed during the later stages of cure, emphasizing the importance of the overall conversion and reactivity of cure [37]. The water produced at the thermoplastic epoxy interface coalesces into a bubble that embeds within the EMAA thermoplastic and remains dormant until activated by heat [38]. When healing is activated at 150 °C, well above the melting point of the EMAA, the bubble expands greatly, pushing the molten thermoplastic into any available damage region binding the fracture surfaces together. While an effective healing agent for the room temperature curing DGEBA/TETA resin system, the low melting point of EMAA of 85 °C has limited the range of epoxy/amine resins evaluated because of the perceived need to ensure that EMAA remains solid and immiscible during the entire cure of the epoxy resin. Indeed, EMAA has been shown to be completely ineffective as a healing agent when cured directly at 150 °C using a DGEBA/diethyl toluene diamine (DETDA) resin system [39]. If a methodology however, could be developed that allowed the healing mechanism to function despite curing the epoxy resin above the melting point of EMAA, then a much wider range of commercial epoxy resin systems could be transformed into mendable epoxy resins regardless of their cure temperature. Meure et al. [37] have shown that when the epoxy/amine resin is added to EMAA at low temperature, below the melting point of the EMAA, the epoxy resin and amine attach to the EMAA surface via non-covalent hydrogen bonding and ionic associations respectively. When the temperature was increased above the melting point of EMAA, bubbles were observed on the surface indicative of the characteristic condensation esterification reaction. It is proposed therefore that encouraging non-covalent attachment of the epoxy/amine to EMAA prior to cure at low temperature may facilitate the condensation mechanism during cure at high temperature and thus healing.
To explore this hypothesis, EMAA modified and unmodified bi-functional DGEBA epoxy resin systems were cured separately with two aromatic amine hardeners, well known for their high performance structural characteristics, namely DETDA and diamino diphenyl sulphone (44 DDS). An initial curing step of 5 h at 80 °C was added to an otherwise single isothermal cure profile of 8 h at 150 °C and compared against a one-step isothermal cure of 8 h at 150 °C. Healing temperature, efficiency and repeatability and fracture toughness of the EMAA modified composites was explored using dual cantilever beam testing (DCB) while the mechanism was investigated using scanning electron microscopy (SEM) and near infra-red (NIR) spectroscopy. A companion study exploring similar concepts, using the aerospace grade tetra glycidyl diamino diphenyl methane (TGDDM) epoxy resin as a high performance and higher cure temperature matrix, is recommended for comparison [40].
Section snippets
Materials
The diglycidyl ether of bisphenol A epoxy resin (DGEBA, EEW = 189, Dow Chemical Company, USA) was degassed in a vacuum oven at 90 °C overnight. The aromatic amines diethyl toluene diamine (DETDA, Albermarle, USA) and 44 diamino diphenyl sulphone (44 DDS, Vantico, USA) were used as received. The poly(ethylene-co-methacrylic acid) (EMAA, Nucrel 2940, DuPont Polymers, USA) pellets were cryogenically ground and then sieved into particles sizes between 295 μm and 495 μm and dried in an air
Controlling epoxy network structure
While the condensation reaction is critical to the effectiveness of the pressure delivery healing mechanism for EMAA [38], it is actually just the final necessary reaction in a sequence of reactions that contribute to the mechanism. Fig. 2 illustrates different available reactions both covalent and non-covalent that all play a role in determining the level of healing. At low temperature, the epoxy resin and amine hardener attaches to the EMAA surface via hydrogen bonding and ionic associations
Conclusion
The use of EMAA as an efficient and repeatable healing agent in a high performance mendable DGEBA/DETDA epoxy composite system has been successfully demonstrated. The key to this development was the design of an appropriate cure profile that included a low temperature cure step (5 h at 80 °C) below the melting point of EMAA (Tm = 85 °C) prior to cure at 150 °C. During this additional cure step, non-covalent hydrogen bonding and ionic associations bind the epoxy resin and amine to the EMAA
Acknowledgements
The authors gratefully acknowledge Mark Greaves for help with obtaining the scanning electron microscopy images of the fractures surfaces.
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2023, Composites Science and TechnologyCitation Excerpt :Poly(ethylene-co-methacrylic acid) (EMAA) is one of the most reported thermoplastic healing agents used in epoxy thermosetting matrices [1–8].