There are increasing concerns regarding the use of solder (as a conductive path) containing lead, a material harmful to humans and the environment, in the electronic industry. In this regard, nano based electrically conductive adhesives (ECAs) consisting of a polymeric resin and conductive nanofillers have been intensively employed to replace harmful metal interconnectors in the electronics industry.
The lower temperature processing, lower cost, lower stress, greater flexibility, creep resistance and energy damping of polymeric materials provide advantages over inorganic eutectics. Up to now, different electrically conductive adhesives based on epoxy resins have been successfully developed and characterized owing to their ease of processing, strong adhesion to a wide range of metals, high tensile strength and modulus, good resistance to heat, chemicals and moisture, and dimensional stability. However, the resin is intrinsically brittle and susceptible to deterioration under crack formation and propagation. Therefore, it cannot be a good option in flexible electronic applications. In contrast to common epoxy-based, two-component or premixed and frozen single-component systems, one part conductive polyurethane adhesives can be efficiently tailored to give a diverse range of products and show more benefits such as flexibility, cost effectiveness, no need for mixing, indefinite pot life with rapid cure at room temperature and a wide application temperature range.
Considering various electrically conductive fillers, graphene related materials have received great research interest for their distinct electrical, optical, thermal, and mechanical properties. Due to the strong intermolecular van der Waals forces existing among graphene nanosheets, achieving homogenous and well exfoliated graphene sheets with single or few layers in both solvents and polymers is challenging. Therefore, physical and chemical surface modification and functionalization of nanosheets through bulk or solution-based chemical approaches have been employed for higher stabilization of dispersed nanomaterials. In this regard, by simple organic reactions, GO is able to covalently bind with polymers or small molecules and after reduction, functionalized graphene nanosheets can be more compatible with polymer matrixes and organic solvents.
Microcracking and hidden damage can lead to structural failures in polymeric matrixes. In such cases, the use of self-healing materials will be very beneficial and effective, because of their great potential for the prevention of polymer degradation and reduction of maintenance costs. The concepts of extrinsic and intrinsic self-healing exist for this process. The extrinsic healing is classified into three main groups: capsule based, vascular, and intrinsic based, in which the embedded healing agents are released after crack formation. In this industry, micro/nanocapsule embedment systems were observed to be one of the most effective approaches for the provision of self-healing ability. Nanocapsules based on polymeric materials can be applied as containers for healants. They can encapsulate a larger quantity of guest substance in their cores and release this on demand at a later stage.
In these nanomaterials, healing is promoted by crack propagation through incorporating capsules in the matrix, which then release their contents into crack surfaces. Isocyanates are a kind of healing material which can develop one-part, catalyst-free self-healing processes while exposed to moist or aqueous environments. In contrast, intrinsic self-healing is based on reversible covalent or noncovalent supramolecular interactions such as π–π interactions or hydrogen bonding and takes place autonomously or with an external trigger like heat or pressure.
In this study, the Iranian researchers reported the synthesis of IPDI loaded PMMA nanocapsules and DIP functionalized graphene nanosheets for achieving the purposes of self-healing and electrical conductivity for polyurethane adhesives, respectively. Polyurethanes with different structures were prepared from polyols (PEG and PCL) and diisocyanates (HDI and TDI) and the conductive nanofillers and nanocapsules as healing materials were further dispersed into polyurethane matrixes in order to prepare the as-mentioned adhesives.
The physical properties of the conductive nanofillers, nanocapsules and adhesives were investigated. In addition, the effect of graphene based nanofillers on the shear strength, thermal stability and electrical conductivity properties of ECAs was studied. The efficiency of the self-healing performance based on the nanocapsules and hydrogen bonding interactions is also demonstrated in this study.
The prepared adhesive has the potential application to be used as a flexible packaging material in power electronic and microwave applications, or as a conductive paste for wearable radio-frequency devices, chip bonding and thermal interface materials.