Flash Joule Heating Equipment: Recyclable multifunctional nanocomposites based on carbon nanotube reinforcement with shape memory and Joule heating capability

Background

Epoxy resins are one of the most commonly used thermosetting resins and have a wide range of applications in industrial sectors such as coatings, adhesives, electronic packaging materials, and advanced composite substrates due to their excellent mechanical properties, dimensional stability, bonding ability, and chemical resistance. The global demand for epoxy resins will continue to grow at a rate of 4-6% over the next five years, and the market size is expected to reach $37.3 billion by 2025. The reported global demand for epoxy resins in different applications includes the most relevant segments such as coatings (50%), composites (18%), construction (13%), and electronics (8%). On the other hand, like other thermoset polymers, conventional epoxy resins cannot be reprocessed, recycled or repaired due to their cross-linked structure. As a result, the volume of epoxy resin waste is increasing dramatically. As a result, economic and environmental factors are driving the development of reprocessable/reformable or/and recyclable epoxy resin systems.

The main point of this paper

Epoxy resin disposal issues:

Traditional epoxy resin disposal methods (landfill and pyrolysis) lead to environmental degradation and waste of resources.

New Epoxy-Based Formulations Needed:

Novel epoxy-based formulations need to be developed for more sustainable remediation, recycling and reprocessing.

Covalent Adaptive Networks (CANs):

Adopt a CANs strategy that combines the benefits of traditional thermosets and thermoplastics to achieve recyclability, remoldability and reprocessability.

Controlled Cleavage Linkages:

Including imine bonding, esterification, Diels-Alder/Retro-Diel-Alders (DA/Retro-DA), disulfide bias, dynamic B-O bonding, and more.

Potential application of CAN as a matrix:

CAN from epoxy monomers is used as a matrix for micro- and nanoscale reinforced materials to improve mechanical, electrical, or thermal properties, and to increase functions such as Joule heating, self-healing, or shape memory.

Application of reinforcing materials:

Use of carbon-derived particles (e.g., CNTs, GNPs, short carbon fibers) to reinforce epoxy matrices, enhance properties, and enable multifunctional micro- or nanocomposites.

Carbon nanotubes enhance the versatility of glass Electrospun Nanofbers

Electrical conductivity vs. CNT content:

As the CNT content increases, the conductivity of the material increases because the number of electrical pathways increases, while the effect of the NH2/epoxy ratio is not significant

Thermo-mechanical properties:

The glass transition temperature (Tg) of the material decreases with increasing NH2/epoxy ratio, and the effect of CNT is more complex due to the lower crosslinking density, which may produce steric hindrance

Joule heating capability:

Joule heating tests show that the material is suitable for resistance heating, with the most conductive samples reaching average temperatures of over 200°C at 100V

Shape memory properties:

Shape memory behavior exhibits high shape fixation (~100%) and high shape recovery (95% under optimal test conditions), and both decreasing the NH2/epoxy ratio and increasing the CNT content impede dynamic bond rearrangement

Chemical recyclability:

Recyclability test results show that the nano-reinforced materials can be recovered and continued to be used, demonstrating the recycling potential of the materials

Summarize

In this research work, multifunctional nanocomposites based on CNT-reinforced glass Electrospun Nanofbers were developed and characterized. In this context, the glass transition temperature, chemical recovery, electrical conductivity, Joule heating and shape memory capacity were analyzed as a function of CNT content and NH2/ epoxy ratio.

Here, an increase in the NH2/ epoxy ratio improves the chemical recovery ability due to the lower crosslinking density, which facilitates the dissolution process and thus allows for the recapture of the CNT with reduced matrix residues.In addition, due to the lower crosslinking density mentioned above, the glass transition temperature decreases when the NH2/ epoxy ratio is increased. However, the glass transition temperature decreases slightly (from 153.4°C to 140.8°C in the worst case) when the NH2/ epoxy ratio is increased, which necessitates the use of higher than stoichiometric cross-linker content in order to obtain a proper chemical cycling.

On the other hand, the addition of carbon nanotubes to the proposed vitreous allows for applications requiring electrical conductivity or Joule heating capacity. Both electrical conductivity and Joule heating capacity increase with increasing carbon nanotube content, which is due to the decrease in electrical resistance. Here, the specimen with the best Joule heating capability reaches an average temperature higher than 100 °C with less than 50 V applied.

Finally, in terms of shape memory properties, all specimens show an excellent shape fixation rate (about 100%). However, when the NH2/ epoxy ratio was increased, the shape recovery rate decreased due to a decrease in the crosslink density and an increase in the dynamic bonding content, which facilitates the rearrangement of the network. However, the increase in CNT content limits the decrease in shape recovery ratio to less than 10% because it hinders the rearrangement of the network.