Nanosized Reinforcements for Adhesives

The development and commercialization of nanoparticles as reinforcement offer new possibilities to tailor adhesives for specific applications. Due to the large surface area of the nanosized particles, only small amounts are needed to cause significant changes in the properties of adhesives. The properties that are generally affected include shear and peel strength, thermal characteristics, and conductivity.

If one would look to the technical literature to see how nanomaterial developments are affecting adhesive formulations, one would see that the focus of the development has been on epoxy matrix systems that are reinforced with carbon nanotubes. This is probably associated to the fact that epoxy / carbon composites are of great interest in high performance applications. However, there are several other nanomaterials that are beginning to see use in adhesive formulations to enhance mechanical as well as other properties. The recent article in SpecialChem4Adhesive.com on POSS enhanced epoxy adhesives is an example of this.

This editorial will look at other nanosized reinforcements that are under investigation and finding their way into early products. Table 1 illustrates the representative types of nanoparticles that are being developed, and this editorial will examine those that have been most studied to date.

The addition of nanofillers causes a significant increase in the mechanical properties of different adhesives. However, the magnitude of enhancement will depend on the type of adhesive, the adherend, joint processing properties, and the specific type and grade of nanofiller that is used for reinforcement. Table 2 shows typical enhancements in shear and peel strength that can be expected of certain nanofillers. An excellent reference on this subject is Chapter 3, "Nanoreinforced Adhesives" by Silvia G. Prolongo, et. al., in Nanofibers, Ashok Kumar, ed., published by INTECH, February 2010.

A group of nanomaterials that has probably generated the greatest expectation is the carbon nanotubes (CNTs). The growing interest of these materials is associated with expectations of spectacular properties. Mechanical properties similar to graphite are possible. Due to their very low diameter and in spite of high stiffness, the carbon nanotubes present very high flexibility. Therefore, they have been investigated as a means of improving fracture toughness of brittle polymers. Carbon nanotubes also have a metal-like character with high thermal and electrical conductivity.

Carbon nanofibers are possible with diameters in the order to 20 - 200 nm and very different lengths from 10 to 100 µm. The expected mechanical properties of carbon nanofibers are lower than those corresponding to carbon nanotubes. However, the nanofibers usually have higher length, and they are also less expensive than CNTs.

Nanoscale alumina fiber powder has also been investigated as a nanofiller. These nanofibers are usually thin (2 - 4 nm in diameter) with a very high aspect ratio and surface area. The addition of alumina nanofibers into epoxy adhesives causes an increase of the peel and strength of the joints of aluminum and composite substrates. However, the effect varies as a function of the manufacturing method applied, depending on whether the adhesive had been bonded to the composite or co-cured with the composite prepreg.

It is also possible to use nanosized particles of silica or aluminum oxide as reinforcement. It is well known that thermophysical and thermomechanical properties of epoxy resins improve considerably due to the addition of nanopowder into the matrix polymer. Also, the addition of low concentrations of these nanoparticles to a typical rubber toughened epoxy adhesive leads to very significant increases in the toughness and shear strength of the resulting joints.