Carbon Nanotube Modified Adhesives

Abstract

Adhesives are becoming increasingly important in the design and operation of a wide variety of fabricated components and products. They being employed more and more to replace or augment more traditional joining techniques such as welding and mechanical fastenings etc. They may be quickly applied, possess excellent properties and are very cost effective. Provided the joints are well designed, the adhesive bonds ought to be one of the strongest aspects of the structure and most certainly will not be the life limiting factor. Nevertheless, there is an ongoing requirement to continuously improve their performance. Being primarily epoxy resins, the development of adhesives has followed that of carbon fibre reinforced epoxy composites. Great strides were made in composite materials in the late 1980s and early 1990s by the dispersion of micron scaled oligomeric thermoplastic materials within the epoxy matrix in order to improve their mechanical. This was done in particular to address the interlaminar toughness issues that blighted carbon fibre aero engine parts in the late 1970s. It was found that the application of similar additions yielded similar advancements in the properties of adhesives. Since around 2010 there has been significant activity in further development of composite performance by the dispersion of nanomaterials, in the forms of graphene, carbon nanotubes and ceramic particles within their matrices. Almost in parallel, a similar approach has been studies with respect to adhesives. The current project looked at the feasibility of improving the performance of two commercial adhesives, with many commercial applications, by the dispersion of Multi Walled Carbon Nanotubes as the “nanofillers”. The work carried out involved the back to back testing of “neat” and modified adhesives in order to assess any improvements. Standard experimental work was carried out using standard laboratory testing, followed up by SEM and also bonded pieces designed to simulate the safety critical load bearing bonded components of recently designed first responder medical evacuation equipment. The latter study being of significance since these are officially certified air borne structures. Two products were investigated; a well established high strength “structural” adhesive and a second, cheaper alternative, considered being “semi-structural”. Bonds between two metal alloys (2 and 6 series aluminium) and between a woven carbon epoxy and a 2-series aluminium were tested. In almost all of the test pieces an improvement was found. Typically values of 6% for the structural adhesive and around 10% for the other. These results were not as high as some of those reported in the literature, but significant nonetheless. The only situation in which an improvement was not observed was in carbon fibre/metal bonding using the structural adhesive when failure occurred by shear in the composite, it being the weakest part of the system. SEM examination of the fracture surfaces showed the addition of the carbon nanotubes to increase the work of fracture within the adhesive which led to the improvement in mechanical properties. It also showed that there is opportunity for further improvement by optimisation of the weight fraction and distribution of the nanotubes. Regardless, the more economic adhesive was demonstrated to have gained enough strength to be considered a “structural” adhesive when modified in this way. As a consequence it has been selected by the sponsoring company as the adhesive of choice in an application that will go into service in early March of this year. Unfortunately that cannot be discussed within the report as it is commercially sensitive.