Yan Chao Yuan

A thermally remendable epoxy resin

To provide epoxy resin with crack healing capability, an epoxy containing both furan and epoxide groups, N,N-diglycidyl-furfurylamine (DGFA), was synthesized through a two-step approach. When it reacted with N,N′-(4,4′-diphenylmethane) bismaleimide (DPMBMI) and methylhexahydrophthalic anhydride (MHHPA), respectively, a crosslinked polymer with two types of intermonomer linkage was yielded. That is, thermally reversible Diels–Alder (DA) bonds from the reaction between furan and maleimide groups, and thermally stable bonds from the reaction between epoxide and anhydride groups. In this way, cured DGFA possessed not only similar mechanical properties as commercial epoxy, but also thermal remendability that enabled elimination of cracks. The latter function took effect as a result of successive retro-DA and DA reactions, which led to crack healing in a controlled manner through chain reconnection.

A dual mechanism single-component self-healing strategy for polymers

A bifunctional single-component healant, glycidyl methacrylate (GMA), is encapsulated and employed for fabricating self-healing epoxy materials. The released GMA is able to rebond cracked portions at room temperature through hydrogen and covalent bonds, and hence recover fracture toughness with high efficiency. The main advances of the healing system lie in the following. (i) It simplifies the conventional approach using two-part healing agent and broadens applicability of the therapy since two healing mechanisms, solvent effect and chemical reactions, are involved. (ii) As GMA contains both epoxide groups and CC bonds, ring-opening and nucleophilic addition reactions between GMA and the residual amine in the matrix occur during crack healing and help to reconnect the separated faces. The application of nucleophilic addition, which has not yet been reported as a healing measure, might lead to expansion of the spectrum of self-healing agent because the species of organic molecules enabling nucleophilic addition reaction are far more than those with specific functional groups like epoxide.

Self-Healing Epoxy Composite with Heat-Resistant Healant

To provide self-healing epoxy composite with adequate heat resistance for high-performance application, we developed a novel microencapsulated epoxy/mercaptan healing agent. The key measure lies in usage of diglycidyl ether of bisphenol A (EPON 828) as the polymerizable component and 2,4,6-tris(dimethylaminomethyl)phenol (DMP-30) as the catalyst. Because of the higher thermal stability of EPON 828 and lower volatility of DMP-30, the healing agent and the self-healing composite not only survive high-temperature curing and thermal exposure, but also offer satisfactory capability of autonomous properties restoration, as characterized by both fracture mechanics and fatigue tests. Especially when the operation temperature is not higher than 200 °C, the performance of the healing system is nearly independent of thermal history.

Self-healing of low-velocity impact damage in glass fabric/epoxy composites using an epoxy–mercaptan healing agent

Self-healing woven glass fabric-reinforced epoxy composite laminates were made by embedding epoxy- and mercaptan-loaded microcapsules. After being subjected to low-velocity impact, the laminates were able to heal the damage in an autonomic way at room temperature. The healing-induced reduction in the damaged areas was visualized using a scanning acoustic microscope. The rate of damage area reduction, which is closely related to the effect of crack rehabilitation and mechanical recovery, is a function of impact energy, content and size of the healing microcapsules. Minor damage, such as microcracks in the matrix, can be completely repaired by the healing system without manual intervention, including external pressure. Microcapsules with larger size and/or higher concentration are propitious for delivering more healing agent to cracked portions, while imposition of lateral pressure on damaged specimens forces the separated faces to approach each other. Both can improve the rate of damage area reduction in the case of severe damage.

Melamine Resin-Walled Microcapsules Containing Styrene: Preparation and Characterization

For purposes of developing a novel self-healing chemistry for polymer composites, melamine-formaldehyde (MF) resin-walled microcapsules containing styrene were prepared by in-situ polymerization in an oil-in-water emulsion. Chemical structure of the microcapsules was identified by Fourier-transform infrared spectroscopy (FTIR) and proton magnetic resonance spectroscopy (1H NMR), respectively. In addition, scanning electron microscope (SEM) and optical microscope (OM) were used to investigate morphology and geometry of the product. The effects of dispersion rate, weight ratio of core to shell and emulsifier concentration were carefully analyzed. It was found that poly(melamine-formaldehyde) (PMF) microcapsules containing styrene were successfully synthesized through the proposed technical route, and their mean diameters fall in the range of 20~71 µm. The rough surface of the microcapsules is composed of agglomerated PMF nanoparticles. Both core content and size of the microcapsule can be adjusted by selecting different processing parameters. The highest loading of styrene in the capsules is about 60% and the emulsifier with lower molecular weight used to result in higher core content. In terms of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), thermal behavior and storage stability of the capsules were studied. The results indicated that the microcapsules can be handled up to 72 oC.

Self-healing woven glass/epoxy composites

Self-healing represents the next generation of technology, which helps to greatly improve important performance of products, including, but not limited to, reliability and durability. This chapter presents an effort of imparting self-healing capability to woven glass/epoxy composites through embedded healing microcapsules. The healing agent consists of epoxy as the polymerizable component and mercaptan or imidazole as the hardener. Upon damaging of the composite materials, the capsules are broken, releasing healing agent, which is delivered to the cracked portions due to capillary effect and then polymerized to rebond the cracks. The results of compression after impact and other characterization techniques show that the damages inside the composites can thus be self-healed at room temperature or elevated temperature by tuning the recipe of healing agent, depending on the application requirements.

Synthesis and Characterization of a Novel Epoxy with Improved Processability

For purposes of developing a novel epoxy with low viscosity and high activity, N,N-diglycidyl-furfurlamine (DGFA) was successfully synthesized through a two-step reaction between 2-furfurylamine and epichlorohydrin involving ring-opening and ring-closure mechanisms. The product structure was verified by FTIR, 1H-NMR, 13C-NMR and elemental analysis, respectively. Its viscosity was found to be 0.02 Pa·s at 25oC. To understand its curing behavior, exothermic habit of the model mixture of DGFA and the curing agent methylhexahydrophthalic anhydride (MHHPA) at stoichiometric ratio of epoxy ring/anhydride of 1:0.8 was inspected with DSC. By changing the heating rates from 2.5 to 15oC/min, activation energy for consolidation of the resin was estimated to be 46.2 kJ/mol, which is much lower than the value involved in curing of diglycidyl ether of bisphenol A catalyzed by anhydride. Besides, thermal decomposition performance of cured version of the newly synthesized epoxy was also examined. The predominant pyrolysis took place at around 330-390oC as a result of chain scission of epoxy. The cured resin possesses comparable mechanical properties as conventional diglycidyl ether of bisphenol A. Its flexural strength and modulus are 111MPa and 3.6GPa, respectively. Evidently, the resultant epoxy is provided with balanced properties for practical applications.