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Posted: Oct 24th, 2006
Nanocomposite adhesives hold enormous potential for demanding applications in displays and electronics
(Nanowerk Spotlight) Adhesives may be broadly divided in two classes: structural and pressure sensitive. To form a permanent bond, structural adhesives harden via processes such as evaporation of solvent or water (white glue), reaction with radiation (dental adhesives), chemical reaction (two part epoxy), or cooling (hot melt). In contrast, pressure sensitive adhesives (PSAs) form a bond simply by the application of light pressure to attach the adhesive to the adherend. PSAs adhere instantly and firmly to nearly any surface under the application of light pressure, without covalent bonding or activation. Waterborne pressure-sensitive adhesives solve the problem of meeting environmental regulations that forbid the emission of volatile organic compounds in manufacturing. However, often waterborne PSAs have poor adhesive performance. Another problem, particularly relevant to display technologies, is how to make an electrically-conducting material that is also flexible and optically transparent. Indium tin oxide is commonly used as a transparent electrode in displays, but it is brittle and prone to mechanical failure or scratching. Adhesives can be made electrically conductive through the addition of metal particles, but then they lose optical transparency, and their adhesiveness is diminished. New research shows that waterborne PSAs containing single-wall carbon nanotubes (SWNTs) meet the requirements of environmental regulations while improving the adhesive performance. The resulting unprecedented combination of adhesion and conductivity properties holds enormous potential for demanding applications in displays and electronics.
To achieve the maximum tack energy (i.e. energy for de-bonding), a PSA must dissipate a large amount of energy on deformation, but it must not have an elastic modulus, E, that is too high. Recent work has shown that using nanocomposite polymer films opens the door to fabricating high performance PSAs.
Keddie explains the new findings to Nanowerk: "The E of poly(butyl acrylate), which is too low to make it a good adhesive, is increased with the addition of SWNTs. At the same time, the amount of energy dissipated increases with SWNT addition. These two factors lead to an optimum SWNT concentration for achieving the maximum tack energy."
The adhesives are optically transparent, as shown here. Looking through an adhesive film, the grass looks just as green! The adhesives have a high adhesion energy, while also being electrically conductive. (Image: T. Wang, University of Surrey)
The blending of colloidal polymer particles and carbon nanotubes enables good mixing at the nanometer length scale. The processing is more simple yet more effective than other methods of making nanocomposites. Typically, one must use aggressive sonic agitation and work at low weight fractions to disperse nanotubes finely enough to obtain homogeneous composites.
"Our colloidal methodology allows for a facile, non-destructive method to create composites with weight fractions as high as 20 wt.%, if required" says Keddie's colleague Dr. Alan Dalton, who leads the Nanostructured and Molecular Materials Group at the University of Surrey.
Previous research has described nanocomposite polymer films made from blends of polymer colloids (i.e. latex) and carbon nanotubes. In some cases, the improvements to mechanical properties were minimal because good dispersion of CNTs was not achieved.
"We dispersed the CNTs in water by grafting a hydrophilic polymer (poly(vinyl alcohol)) onto their surface" says Keddie. "They provide a unique combination of properties. The PSAs are optically transparent while also having electrical conductivity (with a value comparable to germanium) plus a high adhesion energy in comparison to the polymer alone."
In a probe-tack experiment (left), a spherical (or cylindrical) probe in contact with the PSA surface is removed at a constant velocity. The force to de-bond the probe from the surface is measured. The area under the resulting nominal stress/strain curves (right) indicate the total energy of adhesion. PSAs that contain PVA-SWNTs (blue line) have higher tack energies compared to the pure polymer (pink line). The plateau in the curves corresponds to when there is extensive fibrillation during de-bonding. It is apparent that PVA-SWNTs increase the amount of fibrillation. (Graphics: T. Wang, University of Surrey)
The researchers note that the nanocomposite adhesives could find applications in electronics packaging, where metals or semiconductors need to bonded together but electrical and thermal contact must be maintained. Another application area is in displays where transparency and electrical conductivity are required. The nanocomposite could be a possible replacement for indium tin oxide. The fact that the films are a good adhesive could help in the assembly of displays. The layers on either side of the adhesive will adhere without the need for fasteners or bonding.
Keddie and Dalton are in the process of developing nanocomposite PSAs that offer high tack energy on low energy surfaces, such as polyethylene and fluorinated polymers. They also hope to develop PSAs with directional or anisotropic electrical conductivity.
"It has been predicted that carbon nanotubes have an exceptionally-large thermal conductivity and we hope to exploit this property in our PSAs for effective thermal management, which is a key requirement in many industrial applications" says Keddie. "We are also doing some fundamental study to understand the effect of carbon nanotubes on the adhesive de-bonding mechanisms. We are learning how energy is being dissipated at the level of molecules and particles."